Climate

Dictionary of American History
COPYRIGHT 2003 The Gale Group Inc.

CLIMATE

CLIMATE. The climate of an area, defined as the aggregate of weather conditions over time, is constructed from monthly, seasonal, and annual averages of weather elements, such as temperature and precipitation, combined with statements about the frequency of extreme events, such as droughts or tornadoes. Historically, climate has had important economic implications for agriculture, transportation, and settlement. Climatology, or the scientific study of climate, dates to the mid-nineteenth century and includes such specialties as applied climatology, climate dynamics, and climate change.

The classical heritage related the climate of an area uniquely to its latitude. Climate, from the Greek klima, meaning "inclination," was originally thought to depend only on the height of the sun above the horizon, modified in part by special local characteristics. Climate and health have also been closely related throughout history. According to the Hippocratic tradition of ancient Greece, a physician should consider the seasons of the year and what effects each of them produces; the location of a city with respect to winds, waters, terrain, and the rising of the sun; and the particulars of the weather. These were keys to diagnosing and treating diseases in a given location.

The Puzzle of the Early American Climate

Because of its seemingly favorable location in latitudes farther south than most European nations, the New World was expected to have a warm, exotic climate. Initially, colonists and their sponsors envisioned a rich harvest of wine, silk, olive oil, sugar, and spices from their investment. In 1588 the colonial promoter Thomas Harriot pointed out that Virginia was located on the same parallel of latitude as many exotic places, including Persia,

China, and Japan in the East and southern Greece, Italy, and Spain in the West. The reality was much different, however. Early settlers in the Americas found the climate harsher and the storms more frequent and more powerful than in the Old World. In 1644 the Reverend John Campanius of Swedes' Fort, Delaware, wrote of violent winds, unknown in Europe, which tore mighty oaks out of the ground. Another colonist in New Sweden, Thomas Campanius Holm, described rainstorms in which the whole sky was filled with smoke and flames. James Mac Sparran, a missionary to Rhode Island between 1721 and 1757, warned against immigrating to America because the climate was unhealthy, with excessive heat and cold, sudden changes of weather, unwholesome air, and terrible thunder and lightning.

Because of such reports, many Europeans held considerable disdain for the New World and for its climate, soil, animals, and indigenous peoples. The noted Parisian naturalist Georges-Louis Leclerc de Buffon speculated that, because of the cool and humid climate, the flora and fauna of the New World were degenerate. The celebrated botanist and traveler Pehr Kalm observed, rightly or not, that every life-form had less stamina in the New World. People died younger, women reached menopause earlier, soldiers lacked the vitality of their English counterparts, and even the imported cattle were smaller. He pointed to climatic influences as the probable cause.

Citizens in colonial and early America were quite defensive about these opinions and argued that clearing the forests, draining the swamps, and cultivating the land would improve the climate by changing the temperature and rainfall patterns. No general agreement, however, emerged about the direction or magnitude of the change. The Reverend Cotton Mather wrote in the Christian Philosopher (1721) that he believed it was getting warmer. Benjamin Franklin agreed, noting that compared to forested lands, cleared land absorbs more heat and melts snow quicker. In his Notes on the State of Virginia (1785), addressed to a European audience, Thomas Jefferson presented an apology for the harsh American climate and an optimistic prognosis for its improvement by human activities. Hugh Williamson of Harvard College spoke for his generation when he wrote in Observations on the Climate in Different Parts of America (1811) that settlement would result in a temperate climate and clear atmosphere that would serve as "a proper nursery of genius, learning, industry and the liberal arts." In his mind such changes added up to a continent better suited to white settlers and less suited to aboriginal inhabitants.

Climate Observations and Medical Meteorology

The first comprehensive series of meteorological observations in America, taken by John Lining, a physician in Charleston, were related to his medical concerns. In 1740, Lining collected the intake and outflow of his own body for a period of one year in an effort to understand how the weather affected bodily humors and epidemic diseases. Related efforts by Lionel Chalmers, An Account of the Weather and Diseases of South Carolina (1776); William Currie, An Historical Account of the Climates and Diseases of the United States of America (1792); and Noah Webster, A Brief History of Epidemic and Pestilential Diseases (1799) linked regional health conditions to climate and extreme weather events.

Jefferson and the Reverend James Madison began the first simultaneous comparative meteorological measurements in America in 1778. As president of the American Philosophical Society, Jefferson collected weather journals from around the county. He also directed the Lewis and Clark expedition (1804–1806) to take weather observations along the Missouri River and in the Pacific Northwest. Jefferson was a strong advocate for a national meteorological system and encouraged the federal government to supply observers in each county of each state with accurate instruments. Although such a system was not established in his lifetime, many government agencies soon began collecting and compiling observations. During the War of 1812, the surgeon general of the army, James Tilton, ordered the physicians under his command to "keep a diary of the weather" and to file detailed reports on the effects of the climate on the health of the troops. This was because more soldiers were falling ill in camp than were being injured in military engagements. The U.S. Army Medical Department continued its system of taking meteorological measurements at army posts across the country until 1874, in part to document potential changes in the climate. Other early governmental systems included the General Land Office (1817–1821), interested primarily in settlement west of the Appalachian Mountains, and academies in the state of New York (1825–1850), where students collected climatic and phenological statistics. In the 1850s, the U.S. Navy compiled wind and weather charts for the oceans under the direction of Matthew Fontaine Maury.

Under the direction of Joseph Henry, the Smithsonian Institution served as a national center to advance and coordinate meteorological research. The institution conducted storm studies, experimented with telegraphic weather prediction, and collected climate statistics. It also served as a clearinghouse for cooperative observations taken by the navy, the army topographical engineers, the Patent Office, the Coast Survey, the Department of Agriculture, and the government of Canada. Projects completed with Smithsonian data included Climatology of the United States (1857) by Lorin Blodget, Winds of the Globe (1875) by James Henry Coffin, and theoretical studies of the general circulation of the Earth's atmosphere by William Ferrel.

In 1858, Ferrel announced a new theory of fluid mechanics that explained both meridional (E-W) and zonal (N-S) wind flows on the rotating Earth. He wrote equations of motion that accounted for most of the observed features of the general circulation: three vertical circulation cells instead of just one traditional "Hadley cell," high-velocity westerly winds in midlatitudes in both hemispheres, easterly trade winds in the tropics, and low pressure with easterly winds near the poles. Later commentators referred to Ferrel's theory as the "principia meteorologica" because of its fundamental implications for subsequent studies of climate dynamics.

In 1870, Congress established the first national weather service and placed it under the auspices of the Army Signal Office. Colonel Albert J. Myer became the first director of a well-funded national storm warning system employing the nation's telegraphy circuits "for the benefit of commerce and agriculture." In addition to providing daily reports of current conditions and "probabilities" for the next day's weather, the Signal Office collected official climate statistics for the nation. By 1891 the U.S. Weather Bureau had been established in the Department of Agriculture, where it remained until 1940, when it was transferred to the Department of Commerce.

Climate Change in the Nineteenth Century

In 1844, Samuel Forry analyzed data gathered from more than sixty army medical officers and concluded (a) climates are stable and no accurate the rmometrical observations warrant the conclusion of climatic change, (b) climates can be changed by human activity, but (c) these effects are extremely subordinate to physical geography. Elias Loomis studied the temperature of New Haven, Connecticut, and Charles A. Schott constructed national maps of temperature and rainfall. Neither scientist found evidence that humans were changing the climate. Cleveland Abbe, the chief scientist with the National Weather Service, agreed that the old debates about climate change had finally been settled. In an article entitled "Is Our Climate Changing?" published in Forum in February 1889, Abbe defined the climate as "the average about which the temporary conditions permanently oscillate; it assumes and implies permanence."

As the debate over climate change caused by human activities was winding down in the mid-nineteenth century, the discovery that the earth had experienced ice ages produced a plethora of complex but highly speculative theories of climatic change involving astronomical, physical, geological, and paleontological factors. The leading American involved in these discoveries was the prominent glacial geologist T. C. Chamberlin, whose interdisciplinary work on the geological agency of the atmosphere and the effect of carbon dioxide on climate led him to propose a new theory of the formation of the earth and the solar system.

Regional Climates and Identities

Many regions of the United States experience distinctive climatic phenomena. New England, the Appalachian Highlands, and the upper Mississippi Valley have rigorous winters with snow covering the ground, often for several months. The East Coast has a relatively mild climate due to the proximity of the Atlantic Ocean, but these areas are susceptible to land-falling Atlantic hurricanes and

winter "nor'easters." The Deep South has hot summers and mild winters with high humidity because of the proximity of the warm waters of the Caribbean and the Gulf of Mexico; on average this area has the most thunderstorms. The heartland experiences the most violent tornadoes, while the high Plains have the most hailstorms. Monsoonal flows from Mexico water the desert Southwest, while California is susceptible to drying "Santa Anna" winds that can exacerbate wildfires. As scientists have come to realize, all regions of the country may be affected by the El Niño Southern Oscillation of the Pacific Ocean.

It would be foolish to argue that such climatic differences "determined" social relations in these regions, just as it would be futile to argue that the environment made little or no difference to people's lives. It is more productive to ask how the flux of economic and social activities over time changed human relationships with nature in sometimes subtle but often dramatic ways. Horse-drawn sleighs were traditionally safe, enjoyable, and often productive means of winter transportation, yet the widespread use of the automobile transformed snow from a transportation resource into a hazard. Pioneers facing the onset of winter and the possibility of crop failure due to frosts believed that warmer weather was better weather, while contemporary city dwellers in urban heat islands find the weather unbearably hot. Air conditioning undoubtedly stimulated the growth of the Sun Belt, while access to freshwater resources may determine the region's future. In general, social and technological changes and changes in scientific understanding of climate have occurred at much faster rates than have physical changes in the climate system.

Settlers seeking to relocate west of the Appalachian Mountains usually headed due west. They assumed that the climatic zone they were familiar with followed parallels of latitude. Generally, this is not the case, since agricultural hardiness zones gradually slope from northeast to southwest. Thus, for example, settlers from Connecticut established the Western Reserve in Ohio. Further west across the Mississippi River lay the semiarid, treeless prairies that were originally called the "Great American Desert." While farmers on the northern and eastern margins of this area, where annual rainfall totals twenty inches or more, had considerable success, precipitation decreases dramatically to the south and west, attaining true desert conditions in New Mexico and Arizona. The Homestead Act of 1862 encouraged farmers ever westward into marginal lands that were fertile only when it rained. Promoters even resorted to the dubious argument that agriculture somehow increases rainfall, or "rain follows the plow." A succession of drought years could devastate farms, however, as was the case in the decade-long Dust Bowl of the 1930s in the southern Great Plains.

Climate Change in the Twentieth Century

By 1900 most of the chief theories of climate change had been proposed if not yet fully explored: changes in solar output; changes in the earth's orbital geometry; changes in terrestrial geography, including the form and height of continents and the circulation of the oceans; and changes in atmospheric transparency and composition, in part due to human activities. During the International Geophysical Year (1957–1958), Harry Wexler of the U.S. Weather Bureau succeeded in establishing a series of accurate measurements of carbon dioxide. After 1958 these measurements were accurately and faithfully taken at the summit of Mauna Loa volcano in Hawaii by Charles David Keeling. Subsequently, many more international baseline stations have been established. The Keeling curve, the famous saw-toothed curve of rising carbon dioxide concentrations, became the environmental icon of the twentieth century.

In the 1950s, Gilbert Plass developed a computer model of infrared radiative transfer in support of his re-search on carbondioxide and climate. Several years later, in the interest of national security, a climate model known as Nile Blue was developed by the Advanced Research Projects Administration (ARPA) in the Department of Defense. It was hoped that this model could be used to test the sensitivity of the climate to major perturbations, including Soviet tinkering or a major environmental war. In 1967, Syukuro Manabe and Richard T. Wetherald developed a computerized climate model that included the effects of both radiation and convection to calculate temperature as a function of latitude. It predicted a mean warming of 2.3 degrees Celsius for a doubling of carbon dioxide. Two years later, Manabe and Kirk Bryan added basic oceanic features to the model.

The rise of the environmental movement in the early 1970s generated interest in global environmental problems, including climate change. In 1971, when some meteorologists were looking into the possibility of a widespread global cooling, a report from the Study of Man's Impact on Climate conducted at the Massachusetts Institute of Technology returned the focus to carbon dioxide emissions, calling them the largest single anthropogenic change that may influence the climate in the foreseeable future. During this period, anthropogenic effects on climate were called "inadvertent" climate modification. Several other regional and global pollution issues also emerged in the 1970s, including acid deposition and possible damage to the stratosphere by ozone-depleting chemicals and by the exhaust gases of a fleet of supersonic transport planes.

In the 1980s, scientists debated the possibility of a "nuclear winter" caused by an all-out nuclear exchange. Discovery of depleted levels of ozone over Antarctica in 1985 led to the international Montreal Protocol on Substances that Deplete the Ozone Layer, signed in 1987. In 1988 the scientist James Hansen of the National Aeronautics and Space Administration announced to Congress and the world, "Global warming has begun." He went on to report that, at least to his satisfaction, he had seen the "signal" in the climate noise and that the earth was destined for global warming, perhaps in the form of a runaway greenhouse effect. Hansen later revised his remarks, but his statement remained the starting point of widespread concerns over global warming. That same year the Intergovernmental Panel on Climate Change was formed as a joint program of the United Nations Environmental Program, the World Meteorological Organization, and the International Congress of Scientific Unions. It has a mandate to prepare regular assessments of what is known and what should be done about anthropogenic climate change.

The 1992 United Nations Conference on Environment and Development (the Earth Summit) in Rio de Janeiro produced the Framework Convention on Climate Change (FCCC), which calls for a stabilization of atmospheric carbon dioxide concentrations at a level that would prevent human-induced changes in the global climate. The 1997 Kyoto Protocol, calling for legally binding greenhouse gas emission targets for all developed countries, remained a contentious issue in the early twenty-first century. These conventions and protocols represent geopolitical interventions in the climate system. Many more policies were initiated. Economics also began to play a role, as taxes and incentives were put in place to reduce unwanted emissions. Meanwhile, green social engineers attempted to convince the general public to live sustainably, while "geoengineers" hold in reserve massive technical fixes for the climate system. Notably, health issues related to possible climate change returned as policy issues.

Conclusion

The climate issues that puzzled colonists and early Americans were eventually resolved by government-supported scientists who compiled climate statistics for the continent. Changes in human-climate relations were typically caused not by climate change but by people migrating to new regions or by changes in social relations or technology. Anolder medical geography of "airs, waters, and places" was replaced by the germ theory of disease. Yet as Americans gained control of their microclimatic environments through irrigation, central heating, and air conditioning, they began to lose control of the damage they inflicted on the environment, for example, by excessive burning of fossil fuels. In the second half of the twentieth century, new reasons for climate apprehension emerged in the form of local, regional, and global threats to the atmosphere and to human health. By the dawn of the twenty-first century, the social aspects of the climate had grown to encompass scientific, economic, governmental, and diplomatic initiatives regarding the health and future of the planet.

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Climate and the Ocean

Water:Science and Issues
COPYRIGHT 2003 The Gale Group Inc.

Climate and the Ocean

Weather is defined as the state of the atmosphere at a specific place and time, whereas climate is a long-term average of weather in a region. Many factors combine to create the different climates found throughout the world, such as the amount of solar radiation an area receives, local terrain, nearby large bodies of water, and changing geological and biological conditions. Small changes in Earth's orbital pattern around the Sun also can have major effects on climate.

Meteorologists have achieved some success at predicting weather patterns in part because they are of a localized nature and of short duration. Climate, however, takes into account weather factors over a larger region and a longer timespan, and hence is much harder to predict.

Factors Affecting Earth's Climate

The primary factor that affects climate is solar radiation. About half of the Sun's energy radiated towards Earth is absorbed, but this energy is not evenly distributed across the surface. Factors that influence absorption are the transparency of the atmosphere, the angle of the Sun above Earth's surface, and the reflectivity of that surface.

The angle of the Sun above the horizon, known as the angle of incidence, determines the amount of energy striking Earth. If the angle of incidence is high, as it is in the equatorial region, with the Sun nearly perpendicular to Earth's surface, maximum energy will be spread over a small surface area with little reflection. As the angle of incidence drops, as when nearing the poles, the same amount of energy is spread over a much larger area due to the increased angle. More solar energy is reflected out of Earth's system if it comes in at an angle.

The absorption of solar energy is also influenced by Earth's orbital inclination. Because Earth is tilted on its rotational axis 23.5 degrees relative to its orbital plane around the Sun (the ecliptic), middle latitudes of the Northern Hemisphere receive about three times more solar radiation in June than in December. As Earth orbits the Sun, first the Northern Hemisphere, then the Southern Hemisphere is tilted closer to the Sun, creating the seasons. On June 21, the Sun is directly overhead at noon at the Tropic of Cancer. This date is the summer solstice of the Northern Hemisphere. By December 21, southern hemisphere solstice, the Sun is directly over the Tropic of Capricorn at 23.5 degrees south latitude.

This uneven solar heating has created climatic regions of the open ocean that run parallel to the lines of latitude. These climatic regions are relatively stable and are only slightly influenced by surface currents.

Heat Transfer.

If the heat from the Sun were not redistributed, the poles would be much colder and the equator much hotter than they are. Moving currents of air and ocean water redistribute the heat over Earth. Evaporation of water at the equator adds latent heat of vaporization to the atmosphere. The hot, humid air rises at the equator, forming two circulation cells, one on each side.

The influence of Earth's spin causes the velocity at the equator to be much greater than the velocity near the poles. This creates the Coriolis
effect, deflecting moving fluids to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. The resulting complications cause each hemisphere to have three atmospheric circulation cells instead of only one.

The Hadley cells consist of hot air rising at the equator, becoming cooler and denser with movement upward and poleward, and sinking at about 30 degrees north and south latitude. Poleward of the Hadley cells, atmospheric circulation is governed by the Ferrell and Polar circulation cells (see Figure 1). The air rising and sinking at the junctures between these cells governs surface winds and atmospheric pressure across Earth.

Climate Zones

The equatorial region receives the maximum amount of solar radiation. The warm air is capable of evaporating and storing large amounts of water vapor. The warm air begins to rise, causing weak, variable surface winds, known by sailors as the "doldrums." This moist, rising air cools with altitude , generating rain showers almost daily.

Tropical regions extend north and south from the narrow equatorial region to about the Tropic of Cancer and the Tropic of Capricorn, respectively.
This is the area of trade winds, favored by early sailors where consistent winds maintain the strong equatorial currents. Rising heat and water vapor generate the tropical storms and cyclones for which this region is well known.

North and south of the tropics lies a band of hot, dry air known as the subtropical regions. Descending air creates a high-pressure belt with low precipitation, and as in the equatorial region, minimal ocean currents and weak winds. These are the so-called "horse latitudes," where the hot, dry air evaporates the ocean water at an accelerated rate.

The temperate regions lie above 40 degrees North and South latitude. The prevailing westerlies dominate this region with strong winds and unstable weather. As on land, this region is well known for storms of great size and intensity, especially when hot, moist air, as in a tropical cyclone, mixes with the cooler air of the temperate region.

The subpolar regions have predominately low pressure and areas of high precipitation. Sea-surface temperatures reach a summer high of only 5°C (41°F). This allows sea ice to form during winter months and completely cover the ocean until the spring thaw.

The polar regions constantly have high-pressure conditions and very little precipitation. Temperatures rarely rise above freezing and remain below zero most of the year. These regions have the harshest conditions on Earth. Winds seldom cease and the year is divided into six months of light, followed by six months of darkness. Only a few areas in Antarctica briefly escape the lock of the ice.

Climate Change

The study of fossil records indicates that Earth's climate has shifted numerous times in the geologic past. Many plant and animal species have
evolved and then disappeared throughout Earth's past. Climate change has been suggested as a possible cause for some of these mass extinctions.

Climate changes may be caused by several factors: a sudden decrease in amount of available sunlight; variations in Earth's orbit around the Sun; major changes in circulation patterns of the ocean; and changes in the amount of infrared-absorbing greenhouse gases in the atmosphere. The interaction of all these causes, and positive and negative feedbacks among them, make climate predictions very difficult.

Evidence additionally confirms that erupting volcanoes (e.g., Krakatoa, Pinatubo) and impacting asteroids have altered Earth's climate by filling the atmosphere with particulate matter. This is seen in the study of terrestrial outcrops of rock, and of core samples from ocean-floor sediments. If solar energy is severely restricted for an extended period, a drastic change in Earth's climate will result.

Changes in Earth's orbit may create climate changes. The tilt of the rotation axis oscillates between 22.1 and 24.5 degrees over a 40,000-year period. The shape of the orbit changes between an ellipse and a circle over a 100,000 year span. And this spin axis wobbles with an 11,000-year cycle.

Major changes in ocean circulation have important effects on global climate. For example, the geologic closing of the Isthmus of Panama caused a reorganization of currents 4 million years ago. As the Atlantic surface currents pass through the Trade Wind Belt they become saltier by evaporation of water. Instead of moving westward into the Pacific Ocean, the salty water is now blocked by Panama and flows into the North Atlantic. There it is chilled and becomes quite dense, forming the sinking North Atlantic Deep Water that begins the deep-current conveyer belt. If the surface water were fresher, it would be less dense. Instead of sinking, it might flow into polar regions and warm them. Sinking of this water initiated an ice age ; changes in the sinking rate appear to have been closely linked to glacial and interglacial changes in the Northern Hemisphere.

Glacial and Interglacial Periods.

During long periods of cooling, snow and ice could not melt as fast as they accumulated. Over time, glaciers began to form and grow, causing weather changes over the huge ice masses covering the poles. Water evaporated from the oceans and was locked up as snow and ice at the higher latitudes. Because Earth holds only a finite amount of water, ocean levels began to drop. At the height of the last glacial age about 18,000 years ago, the oceans may have been as low as 150 meters (500 feet) below their present level.

Warming climates would melt glacial ice faster than it was being created, slowly recharging the ocean basins. These periods of interglaciation generally exhibited mild enough conditions to push back the polar glaciers and allow for migration and distribution of both marine and terrestrial species, including humans.

Global Warming.

Global warming will be a point of research and debate for the foreseeable future. Global warming is part of a natural cycle in the broader climate-change scenario, which is confirmed in the fossil and geologic record. However, human activity has had an impact that, if not
causing global warming, is at least helping to accelerate it. Records of atmospheric CO2 in glacial ice over time show a correlation between high CO2 content and warming of the global climate.

Researchers agree that global warming will produce changes, but do not agree as to what exactly those changes will be or their intensity. An increase in tropical storms, heat waves, and precipitation has been suggested. The rise in global temperatures may influence the ocean's deep-water circulation patterns, which can cause rapid climate change, which in turn would affect the global distribution of plant and animal species.

Another possible change would be accelerated melting of the polar ice caps. Water released from the melting ice would cause a rise in sea level, flooding low-lying coastal areas.

see also Carbon Dioxide in the Ocean and Atmosphere; El NiÑo and La Niña; Glaciers, Ice Sheets, and Climate Change; Global Warming and the Hydrologic Cycle; Global Warming and the Ocean; Ice at Sea; Ocean Currents; Oceans, Polar; Oceans, Tropical; Weather and the Ocean.

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Climate and Food

Encyclopedia of Food and Culture
COPYRIGHT 2003 The Gale Group Inc.

CLIMATE AND FOOD

CLIMATE AND FOOD. Throughout most of prehistory, humans acquired food by hunting, fishing, gathering, foraging, or scavenging. The animals and plants they consumed were native to the local climate and environment and provided highly variable diets. Arctic and subarctic populations fished, gathered shellfish, and hunted land and sea mammals; temperate forest populations gathered seasonal plants and hunted wildlife; prairie and savanna dwellers hunted and trapped large mammals; and tropical forest dwellers fished, gathered a variety of plant foods, and hunted small mammals. Climatic influences on the flora and fauna included in the local diet were rainfall, temperature, seasonality, and longer-term cooling and warming trends. The most extreme climate changes were the Pleistocene glacial advances and retreats in the northern hemisphere. Climatic variation in temperature and precipitation became central to food procurement when plants and animals were first domesticated about ten thousand years ago at the end of the Pleistocene epoch.

There are only a few small populations that subsist entirely on hunting and gathering of wild plants and animals, although many populations continue to supplement their diets with wild foods. An exception to this is the fish and shellfish that provide substantial amounts of food to people through commercial fishing. Nevertheless, most peoples around the globe consume domestic plants and animals that are grown or raised locally or are produced commercially.

Climate and World Biomes

Climate (general, longer-term) and weather (specific, short-term) tend to structure ecosystems around the world by regulating rates of plant photosynthesis and production and contributing to patterns of vegetation and animal life. The principal factors are temperature, which is largely a function of global latitude and elevation; precipitation, drainage, and stored fresh water resources; windflow, which can dry or chill; solar radiation; and seasonal patterns in all of these, particularly temperature and rainfall. Polar and subpolar ecosystems, which are unsuitable for agriculture, are characterized by cold winters, cool summers, and limited precipitation. Some livestock are kept in polar and mountain ecosystems: llama and alpaca in the Andes, yak in the Himalayas, reindeer in the Arctic. Temperate and subtropical zone ecosystems may have relatively high precipitation, marked seasonality in temperature, and agricultural growing seasons up to six months. Drier temperate continental ecosystems (prairies, steppes) are also highly seasonal in rainfall with cold winters. Relatively dry, temperate zones can be highly productive with the practice of irrigation. Mediterranean ecosystems (including California, Chile, and parts of the Near East) have cool, wet winters and hot, dry summers. Many of the major cereal crops of the world were domesticated in these seasonally dry, temperate, or Mediterranean ecosystems: maize or corn in Middle America, wheat and barley in the Near East, rice in Asia, and sorghum in Africa. Quinoa, a member of the goosefoot family, was domesticated in the cool, seasonally dry reaches of the Andes. Tropical ecosystems have warm temperatures throughout the year but often with seasonality of rainfall. Those ecosystems with limited and seasonal rainfall grade into tropical grazing lands or savanna, while increased rainfall yields forests from sparse woodlands up to densely wooded rainforests. Widespread rainforest agriculture today includes a form of shifting, swidden, or slash-and-burn cultivation. Within each of these broadly-defined ecosystems or biomes, there is considerable
variation: variation by season and by year, with inherent risks to livestock and agricultural production. For example, dramatic heat waves or cold periods, droughts, floods, hailstorms, and hurricanes can destroy crops and domestic livestock, producing a loss in food security and even famine. These extreme events have a major impact when they occur in heavily populated areas.

Plants and animals were first domesticated in the seasonally dry Mediterranean climate of the Fertile Crescent in the Near East. These farming and livestock practices then spread along the Eurasian east-west axis zone of similar latitude and climate. Most domestic seed plants (e.g., cereals, goosefoots) and pulses (e.g., beans, lentils, grams, peas) were temperate-zone domesticates, whereas some tubers and root crops were domesticated in the tropics (e.g., manioc, yams, taro). With the discovery of the New World by Europeans, many native American foods spread to parts of the Old World: the potato became a staple in temperate zones of Europe and the Himalayas; maize became a staple in the drier African tropics; manioc became a staple in the wetter African tropics. Other New World temperate-zone domesticates, such as chocolate, peanuts, and tomatoes, became favored foods around the globe.

Today, temperate and subtropical agroclimatic zones of the United States, Argentina, Europe, and eastern Asia (China and Japan) still have the highest productivity of domestic grains and livestock that feed a substantial portion of the world. This results from a favorable combination of sophisticated agrotechnology and climate. Figure 1 illustrates how climatic inputs interact with the flows of information and resources in a Western industrialized system of agriculture.

Temperate and subarctic marine biomes are highly productive sources of fish and shellfish, although these food resources are in decline because of effective commercial exploitation by Western nations.

Food Intake and Climate

Some patterns of food intake are indirectly or directly linked to climate. For example, tropical populations are often limited in protein intake. Solomon Katz (1987) noted that this occurs in traditional agricultural populations dependent on grains (maize, rice, sorghum, millet) or tubers (potato, manioc) that are high in calories, but relatively lower in protein. Among tropical forest dwellers, as in the Amazon and the Congo basins, protein must come from fish, insects, some game animals, and plant foods. A direct effect of climate is the high metabolic need for calories in arctic or subarctic zones and temperate zone winters because of increased energy needs for temperature regulation in the cold. Derek Roberts (1978) documented that arctic dwellers have an elevated basal metabolic rate (BMR), which may be adaptive in the cold. Infants who are kept under cool conditions have higher food calorie requirements for normal weight gain than infants kept under warmer conditions. In Western industrialized nations, reduced activity levels during the winter season lead to unhealthy increases in the accumulation of human body fat (and weight) or energy storage. On the other hand, the accumulation of body fat in Ama women who dive for edible seaweed throughout the year allows them to withstand the cold water off the shores of Korea and Japan.

An alarming trend that is certain to influence human patterns of food intake is recent climate change. Some variation in weather and climate is normal. Yet within the past 250 years, however, increased atmospheric carbon dioxide (CO2), resulting from fossil fuel combustion, deforestation, and agricultural activities, has led to a "green-house" effect and global warming. A major compilation of research by Houghton and other scientists from the Intergovernmental Panel on Climate Change (IPCC) in 2001 has demonstrated beyond any reasonable doubt that human activities have produced a 1.1°F (0.6°C) rise in average global temperature over the past 150 years (see Figure 2). And by the year 2100, this global temperature is expected to rise another 1.8 to 6.3°F (1.0 to 3.5°C), a change that is greater than any experienced on the globe within the past ten thousand years.

Global warming will have variable effects on local weather and climate that are dependent on latitude, elevation, and geographic location. For example, McCarthy and others (2001) have shown that sea level rise from melting glaciers during the twentieth century has been about 6 inches (15 cm), and a projected rise during the twenty-first century is an additional 18.9 inches (48 cm). This will contribute to a loss of coastal agricultural lands and an increased salinization of water and coastal lands. Influences on agricultural food production are likely to be pronounced. Higher temperatures will cause rises in rainfall and the likelihood of floods in some areas and declines in rainfall and consequent drought in other areas: extremes in weather events (floods, hurricanes, heat waves, droughts) will be more common. Both conditions will lead to crop losses and decreased plant productivity. There will be increased heat stress in livestock leading to lower milk and meat production. At the same time that coastal agricultural and grazing land will be lost to sea level rise and salinization, the human population will continue to increase, putting greater pressure on food resources.

It is estimated that the impacts of global warming will be greatest in those regions of the world such as Asia, Africa, Latin America, and the Pacific Islands, where the adaptive capacity is low and vulnerability is high because of the lack of economic resources. Africa is likely to be especially hard hit because such a large part of its land resources is arid or semi-arid savanna lands. Of the total desertification and degradation around the globe, nearly
30 percent is in Africa. Although the debate continues on whether overgrazing, overpopulation, or warming trends are the cause of desertification, nevertheless, global warming will certainly increase the expanse of dry lands on this continent and elsewhere.

Humans have a remarkable capacity to adapt to change, including climate change, through culture and technology. Global warming and its consequent negative effects on our capacity to produce food will be an unprecedented challenge to this adaptability.

Katz, Solomon H. "Food and Biocultural Evolution: A Model for the Investigation of Modern Nutritional Problems." In Nutritional Anthropology, edited by Francis E. Johnston. New York: Alan R. Liss, 1987.

McCarthy, J. J., et al., eds. Climate Change 2001: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, U.K.: Cambridge University Press, 2001.

National Research Council. Climate and Food: Climatic Fluctuation and U.S. Agricultural Production: A Report on Climate and Weather Fluctuations and Agricultural Production. Board on Agriculture and Renewable Resources, Commission on Natural Resources, National Research Council. Washington, D.C., National Academy of Sciences, 1976.

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Climate

Encyclopedia of the Modern Middle East and North Africa
COPYRIGHT 2004 The Gale Group, Inc.

CLIMATE

middle eastern climatic conditions vary greatly, depending on the season and the geography.

The Middle East and North Africa are perceived as both homogeneous and intensely arid, but the region is best characterized by its climatic variation. Although the hot arid, or desert, climate predominates in the region, the well-watered highlands of Turkey and the mountains of Iran and Ethiopia are important as sources of the region's major rivers. Climatic variation finds further expression in the temperature regimes of the northern and southern parts of the area. Average July maxima for inland

locations near 30° north latitude are as high as 108°F (42°C), while summer maximum temperatures in northern locations such as Ankara, Turkey, do not exceed 86°F (30°C). Black Sea coastal stations' (e.g., Trabzon, Turkey) average summer maxima may be as low as 79°F (26°C). January average minimum temperatures fall to 50°F (10°C) in Aswan, but reach 10°F (—12.5°C) in Erzurum on the Anatolian plateau.

Desert conditions are primarily the result of the subtropical zone of high pressure that coincides with 30° north latitude. In this area, cold, subsiding air warms as it approaches the earth, thus increasing its ability to hold moisture. This results in extreme evaporation from all surfaces, and under such conditions, very little rain falls. During the summer solstice, the sun is directly overhead at 23° 30′ at north latitude (e.g., at Aswan, Egypt). Annual periods of high sun in combination with clear skies through much of the year allow intense solar radiation with subsequent extreme evapotranspiration demands.

Evapotranspiration refers to the water needed by vegetation to withstand the energy of incoming solar radiation. This is accomplished through the mechanism of heat transfer by means of evaporation from inert surfaces and transpiration from stomata (pores) on leaf surfaces. Total demands made upon an individual plant are termed potential evapotranspiration (PE). Actual evapotranspi-ration (AE) is the amount of water actually available and used by the plant and reflects climatic conditions rather than optimal plant requirements. The difference between PE and AE defines the degree of aridity or drought and also the amount of irrigation water that would have to be applied for such vegetation to survive.

In the deserts of North Africa and Southwest Africa, total annual precipitation is between 2 inches (50 mm) and 14 inches (350 mm). The area from Aden to Baghdad receives from less than 2 inches (50 mm) annually to about 6 inches (150 mm). More than 39 inches (1 m) of water would be required in those places to sustain rain-fed agriculture. Under such conditions, sparse natural vegetation allows animals some seasonal grazing at best. Hyperarid areas, which seldom if ever receive rain, have no vegetation at all. Rainfall variability within the area of desert climate exceeds 40 percent, reducing to 20 percent on the moist margins of the semiarid zone, which forms a transition between the true desert to the south and the more humid areas farther north.

Precipitation on the semiarid margins of Middle Eastern deserts ranges from 14 inches (350 mm) to 30 inches (750 mm) annually. Dry farming of grains employing alternate years of fallow can be carried out with 16 inches (400 mm) or more of rain. It should be remembered that, while rainfall variability is greatest in the desert, this also means that aridity there has high predictability. Thus, the semiarid transition between regions of predictable aridity and predictable rainfall is one where rain-fed agriculture is possible but has a high chance of failure. This is biblical country—years of plenty followed by years of famine—and one to which pastoral nomadism was a practical adaptation.

The Black Sea coast of Turkey receives from 78 inches (2,000 mm) to 101 inches (2,600 mm) per year, although the transition from the windward, watered side of the Pontic range to the leeward, dry side can be very abrupt due to the topography. The Mediterranean climate, which is limited to a narrow coastal strip reaching from Gaza to Istanbul and from Tunis in the west to the Atlantic, is marked by mild winters with ample rain and long, hot summers when Sahara-like conditions prevail.

Precipitation results from three different processes. Orographic precipitation occurs on the Pontic and Taurus mountains of Turkey; the Elburz and Zagros mountains of Iran; the peaks of Lebanon and the hills of Israel, the West Bank, and Jordan; the highlands of Ethiopia; and the Atlas and Anti-Atlas mountains of northwest Africa. Such precipitation occurs as warm, moisture-bearing winds are forced to higher elevations over the mountains. When the air cools, it loses its ability to hold moisture, and rain or snow falls on the wind-ward sides of those ranges.

The Anatolian plateau and the steppes of northern Syria experience small quantities of rain in the form of convectional summer showers from thunderstorms. Equatorial convectional rains provide the waters of the White Nile.

A third cause of precipitation, particularly in the wintertime, is the passage of frontal systems from west to east across the region bringing alternating high and low pressure cells with associated cold, clear, or moist warm air masses. Frontal systems are propelled eastward by the subtropical jet stream, the position of which varies latitudinally by as much as 15° from a winter position in the north to its summer position in the south. Summer months find the path of the jet stream located from central Turkey northeastward to central Asia. Six months later the jet stream is at its maximum along a path traced across the Gulf of Suez to the head of the Gulf of Aqaba and beyond. This shift accounts for the changes in temperature and precipitation noted above.

Surface winds in the Middle East have distinctive qualities and have received local names famous throughout the region. The cold northern wind blowing from the Anatolian plateau to the southern Turkish shore in the winter is the Poyraz (derived from the Greek: bora, i.e., north); the warm on-shore wind in the same location is known as the meltem. Searing desert winds are infamous: The Egyptian khamsin, which blows in from the desert, is matched by the ghibli in Libya and the simoon in Iran.

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Climate

Encyclopedia of Russian History
COPYRIGHT 2004 The Gale Group Inc.

CLIMATE

There are five climates in Russia. The Polar climate hugs the Arctic coast and yields a 60-day growing season, with July and August averaging temperatures over freezing. Tundra vegetation prevails. South of the tundra, blanketing two-thirds of Russia, is the Subarctic climate with its brief cool summers and harsh cold, mostly dry, winters. With a growing season of sixty to ninety days, the dominant vegetation is taiga (northern coniferous forest), 90 percent of which is underlain by permafrost. In European Russia and Western Siberia, the Subarctic climate merges southward with the Humid Continental Warm-to-Cool Summer climate. Here the winters become less harsh, although much snowier, and the summers become longer and warmer. Growing seasons reach 90 to 120 days, and the vegetation is a temperate mixed forest that joins the broadleaf forests and grasslands to the south. Along the Lower Volga and in the North Caucasus, the climate becomes sub-humid. Here summers become equal to winters, a Semiarid Continental climate prevails, the growing season reaches 120 to 160 days, and grassland, or steppe, vegetation dominates. Particular to Russia's "bread-basket," a terrain with rich black loams, this climate suffers from insufficient precipitation. A tiny strip of Arid Continental climate fringes the Russian shoreline of the Caspian Sea. With hot, dry summers and rather cold, shorter winters, this climate yields a 160- to 200-day growing season. A true desert, it reflects a severe soil-moisture deficit.

Russia's massive landmass, northerly location, and flat-to-rolling terrain dramatically influence these climates. Because three-fourths of Russia is more than 250 miles (400 kilometers) away from a largely frozen sea, the climates are continental, not maritime. Continental climates exhibit wide ranges of temperature (the difference between the warmest and coldest monthly averages) and low average annual precipitation that peaks in summer instead of spring. Climatic harshness increases from west to east as the moderating influence of the warm North Atlantic Ocean decreases. St. Petersburg on the Gulf of Finland has a 45° F (25° C) difference between the July and January mean temperatures and 19 inches (48 centimeters) of annual precipitation. Yakutsk in Eastern Siberia contrasts with a 112° F (65° C) range of temperature and only 4 inches (10 centimeters) of precipitation.

Russia's high-latitude position enhances continentality. Nine-tenths of the country is north of 50° N Latitude. Moscow is in the latitude of Edmonton, Alberta; St. Petersburg equates with Anchorage, Alaska. Russia thus resembles Canada in climate more than it does the United States. High-latitude countries like Russia and Canada suffer low-angle (less-intense) sunlight and short growing

seasons that range from 60 days in the Arctic to 200 days along the Caspian shore.

Low relief also augments the negative effects on Russia's climates. Three-fourths of Russia's terrain lies at elevations lower than 1,500 feet (450 meters) above sea level. This further diminishes the opportunities for rain and snow because there is less friction to cause orographic lifting. The country's open western border, uninterrupted except for the low Ural Mountains, permits Atlantic winds and air masses to penetrate as far east as the Yenisey River. In winter, these air masses bring moderation and relatively heavy snow to many parts of the European lowlands and Western Siberia. Meanwhile, a semi-permanent high-pressure cell (the Asiatic Maximum) blankets Eastern Siberia and the Russian Far East. This huge high-pressure ridge forces the Atlantic air to flow northward into the Arctic and southward against the southern mountains. Consequently, little snow or wind affects the Siberian interior in winter. The exceptions are found along the east coast (Kamchatka, the Kurils, and Maritime province).

As the Eurasian continent heats faster in summer than the oceans, the pressure cells shift position: Low pressure dominates the continents and high pressure prevails over the oceans. Moist air masses flow onto the land, bringing summer thunderstorms. The heaviest rains come later in summer from west to east, often occurring in the harvest seasons in Western Siberia. In the Russian Far East, the summer monsoon yields more than 75 percent of the region's average annual precipitation. Pacific typhoons often harry Kamchatka, the Kurils, and Sakhalin Island.

Winter temperatures plunge from west to east. Along Moscow's 55th parallel, average January temperatures fall from a high of 22° F (–6° C) in Kaliningrad to 14° F (–10° C) in the capital to 7° F (–14° C) in Kazan to–6° F (–20° C) in Tomsk. Along the same latitude in the Russian Far East, the temperatures reach low averages of–29° F (–35° C). Northeast Siberia experiences the lowest average winter temperatures outside of Antarctica:–50° F (–45° C), with one-time minima of–90° F (–69° C).

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climate

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

climate, average condition of the atmosphere near the earth's surface over a long period of time, taking into account temperature, precipitation (see rain), humidity, wind, barometric pressure, and other phenomena.

Primary Influence on Climate

The major influence governing the climate of a region is its latitude. A broad latitudinal division of the earth's surface into climatic zones based on global winds includes the equatorial zone, or doldrums, characterized by high temperatures with small seasonal and diurnal change and heavy rainfall; the subtropical, including the trade-wind belts and the horse latitudes, a dry region with uniformly mild temperatures and little wind; the intermediate, the region of the prevailing westerlies that, because of several secondary influences, displays wide temperature ranges and marked changeability of weather; and the polar, a region of short summers and long winters, where the ground is generally perpetually frozen (see permafrost). The transitional climate between those of the subtropical and intermediate zones, known as the Mediterranean type, is found in areas bordering the Mediterranean Sea and on the west coasts of continents. It is characterized by mild temperatures with moderate winter rainfall under the influence of the moisture-laden prevailing westerlies and dry summers under the influence of the horse latitudes or the trade winds.

Secondary Influences on Climate

The influence of latitude on climate is modified by one or more secondary influences including position relative to land and water masses, altitude, topography, prevailing winds, ocean currents, and prevalence of cyclonic storms. Climatic types combining the basic factor of latitude with one or more secondary influences include the continental and the marine. Except in the equatorial region, the continental type is marked by dry, sunny weather with low humidity and seasonal extremes in temperature; noteworthy are the Sahara and Siberia. Marine climates are characterized by small annual and diurnal temperature variation and by copious rainfall on the windward side of coastal highlands and mountainous islands; notable is the mean annual precipitation of 451 in. (1146 cm) at Mt. Waialeale, Hawaii.

The coastal, or littoral, climate is one in which the direction of the prevailing winds plays a dominant role—the east coasts having generally the heavier rainfall in the trade-wind belts, the west coasts in westerly belts. Both coasts have a climate resembling the continental during the season when the wind is blowing from the interior of the continent. An instance of the coastal type, in which the precipitation is accentuated by the nearness of a mountain barrier, is the west coast of North America from Alaska to Oregon, where the mean annual precipitation averages 80 to 100 in. (203 to 254 cm), almost all of it falling during the winter months. Elevation is the dominant factor in mountain and plateau climates, with the temperature decreasing about 3°F per 1,000 ft (1.7°C per 305 m) of ascent and rainfall increasing with altitude up to about 6000 ft (1829 m), then decreasing with further elevation.

Climatology and Climatic Change

Climatology, the science of climate and its relation to plant and animal life, is important in many fields, including agriculture, aviation, medicine, botany, zoology, geology, and geography. Changes in climate affect, for example, the plant and animal life of a given area. The presence of coal beds in North America and Europe along with evidence of glaciation in these same areas indicates that they must have experienced alternately warmer and colder climates than they now possess.

Despite yearly fluctuations of climatic elements, there has been, apparently, little overall change during the period of recorded history. Numerous climatic cycles (variations in weather elements that recur with considerable regularity) have been claimed to exist, including an 11-year cycle related to sunspot activity. There is currently much concern that human activities are changing the earth's climate in harmful ways. Computer models of climate changes have been developed in recent years; some examine potential parameters that effect global warming or cooling.

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the branch of biology that studies the relation between variations in climate and periodic biological phenomena, as the migration of birds or the flowering of plants. —phenologist , n. —phenologic, phenological , adj.

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climate

cli·mate
/ ˈklīmit/
•
n.
the weather conditions prevailing in an area in general or over a long period:
our cold, wet climate. ∎
a region with particular prevailing weather conditions:
vacationing in a warm climate. ∎
the prevailing trend of public opinion or of another aspect of public life:
the current economic climate.DERIVATIVES:cli·mat·ic
/ klīˈmatik/ adj.cli·mat·i·cal
/ klīˈmatikəl/ adj.cli·mat·i·cal·ly
/ klīˈmatik(ə)lē/ adv.ORIGIN:late Middle English: from Old French climat or late Latin clima, climat-, from Greek klima ‘slope, zone,’ from klinein ‘to slope.’ The term originally denoted a zone of the earth between two lines of latitude, then any region of the earth, and later, a region considered with reference to its atmospheric conditions. Compare with clime.

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climate

climate The characteristic pattern of weather elements in an area over a period. The weather elements include temperature, rainfall, humidity, solar insolation, wind, etc. The climate of a large area is determined by several climatic controls: (1) the latitude of the area, which accounts for the amount of solar radiation it receives; (2) the distribution of land and sea masses; (3) the altitude and topography of the area; and (4) the location of the area in relation to the ocean currents. Weather elements are important abiotic factors.

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climate

climate Weather conditions of a place or region prevailing over a long time. The major factors influencing climate are temperatures, air movements, incoming and outgoing radiation and moisture movements. Climates are defined on different scales, ranging from macroclimates, which cover the broad climatic zones of the globe, down to microclimates, which refer to the conditions in a small area, such as a wood or a field.

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Climate

Environmental Encyclopedia
COPYRIGHT 2003 The Gale Group Inc.

Climate

Climate is the general, cumulative pattern of regional or global weather patterns. The most apparent aspects of climate are trends in air temperature and humidity, wind, and precipitation. These observable phenomena occur as the atmosphere surrounding the earth continually redistributes, via wind and evaporating and condensing water vapor, the energy that the earth receives from the sun.

Although the climate remains fairly stable on the human time scale of decades or centuries, it fluctuates continuously over thousands or millions of years. A great number of variables simultaneously act and react to create stability or fluctuation in this very complex system. Some of these variables are atmospheric composition, rates of solar energy input, albedo (the earth's reflectivity), and terrestrial geography. Extensive research helps explain and predict the behavior of individual climate variables, but the way these variables control and respond to each other remains poorly understood. Climate behavior is often likened to "chaos," changes and movements so complex that patterns cannot be perceived in them, even though patterns may exist. Nevertheless, studies indicate that human activity may be disturbing larger climate trends, notably by causing global warming. This prospect raises serious concern because rapid anthropogenic climate change could severely stress ecosystems and species around the world.

Solar energy and climate

Solar energy is the driving force in the earth's climate. Incoming radiation from the sun warms the atmosphere and raises air temperatures, warms the earth's surface, and evaporates water, which then becomes humidity, rain, and snow. The earth's surface reflects or re-emits energy back into the atmosphere, further warming the air. Warming air expands and rises, creating convection patterns in the atmosphere that reach over several degrees of latitude. In these convection cells, low pressure zones develop under rising air, and high pressure zones develop where that air returns downward toward the earth's surface. Such differences in atmospheric pressure force air masses to move, from high to low pressure regions. Movement of air masses creates wind on the earth's surface. When these air masses carry evaporated water, they may create precipitation when they move to cooler regions.

The sun's energy comes to the earth in a spectrum of long and short radiation wavelengths. The shortest wavelengths are microwaves and infrared waves. Infrared radiation is felt as heat. A small range of medium wavelength radiation makes up the spectrum of visible light. Longer wavelengths include ultraviolet (UV) radiation and radio waves. These longer wavelengths cannot be sensed, but UV radiation can cause damage as organic tissues (such as skin) absorb them. The difference in wavelengths is important because long and short wavelengths react differently when they encounter the earth and its atmosphere.

Solar energy approaching the earth encounters filters , reflectors, and absorbers in the form of atmospheric gases, clouds, and the earth's surface. Atmospheric gases filter incoming energy, selectively blocking some wavelengths and allowing other wavelengths to pass through. Blocked wavelengths are either absorbed and heat the air or scattered and reflected back into space. Clouds, composed of atmospheric water vapor, likewise reflect or absorb energy but allow some wavelengths to pass through. Some energy reaching the earth's surface is reflected; a great deal is absorbed in heating the ground, evaporating water, and conducting photosynthesis . Most energy that the earth absorbs is re-emitted in the form of short, infrared wavelengths, which are sensed as heat. Some of this heat energy circulates in the atmosphere for a time, but eventually it all escapes. If this heat did not escape, the earth would overheat and become uninhabitable.

Variables in the climate system

Climate responds to conditions of the earth's energy filters, reflectors, and absorbers. As long as the atmosphere's filtering effect remains constant, the earth's reflective and absorptive capacities do not change, and the amount of incoming energy does not vary, climate conditions should stay constant. Most of the time, though, some or all of these elements fluctuate. The earth's reflectivity changes as the shapes, surface features, and locations of continents change. The atmosphere's composition changes from time to time, so that different wavelengths are reflected or pass through. The amount of energy the earth receives also shifts over time.

During the course of a decade the rate of solar energy input varies by a few watts per square meter. Changes in energy input can be much greater over several millennia. Energy intensity also varies with the shape of the earth's orbit around the sun. In a period of 100 million years the earth's elliptical orbit becomes longer and narrower, bringing the earth closer to the sun at certain times of year, then rounder again, putting the earth at a more uniform distance from the sun. When the earth receives relatively intense energy, heating and evaporation increase. Extreme heating can set up exaggerated convection currents in the atmosphere, with extreme low pressure areas receiving intensified rains and high pressure areas experiencing extreme drought .

The earth's albedo depends upon surface conditions. Extensive dark forests absorb a great deal of energy in heating, evaporation of water, and photosynthesis. Light, colored surfaces, such as desert or snow, tend to absorb less energy and reflect more. If highly reflective continents are large or are located near the equator, where energy input is great, then they could reflect a great deal of energy back into the atmosphere and contribute to atmospheric heating. However, if those continents are heavily vegetated, their reflective capacity might be lowered.

Other features of terrestrial geography that can influence climate conditions are mountains and glaciers. Both rise and fall over time and can be high enough to interrupt wind and precipitation patterns. For instance, the growth of the Rocky Mountains probably disturbed the path of upper atmospheric winds known as the jet stream. In southern Asia, the Himalayas block humid air masses flowing from the south. Intense precipitation results on the windward side of these mountains, while the downwind side remains one of the driest areas on Erth.

Atmospheric composition is a climate variable that began to receive increased attention during the 1980s. Each type of gas molecule in the atmosphere absorbs a particular range of energy wavelengths. As the mix of gases changes, the range of wavelengths passing through the filter shifts. For instance, the gas ozone (O3) selectively blocks long wave UV radiation. A drop in upper atmospheric ozone levels discovered in the late 1980s has caused alarm because harmful UV rays are no longer being intercepted as effectively before they reach the earth's surface. Water vapor and solid particulates (dust) in the upper atmosphere also block incoming energy. Atmospheric dust associated with ancient meteor impacts is widely thought responsible for climatic cooling that may have killed the earth's dinosaurs 65 million years ago. Climate cooling could occur today if bombs from a nuclear war threw high levels of dust into the atmosphere. With enough radiation blockage, global temperatures could fall by several degrees, a scenario known as nuclear winter .

A human impact on climate that is more likely than nuclear winter is global warming caused by increased levels of carbon dioxide (CO2) in the upper atmosphere. Most solar energy enters the atmospheric system as long wavelengths and is reflected back into space in the form of short wavelength (heat) energy. Carbon dioxide blocks these short, warm wavelengths as they leave the earth's surface. Unable to escape, this heat energy remains in the atmosphere and keeps the earth warm enough for life to continue. However, many studies suggest that the burning of fossil fuels and biomass have raised atmospheric carbon dioxide levels. Rising CO2 levels could trap excessive amounts of heat and raise global air temperatures to dangerous levels. This scenario is popularly known as the greenhouse effect . Extreme amounts of trapped heat could disturb precipitation patterns. Ecosystems could overheat, killing plant and animal species. Polar ice caps could melt, raising global ocean levels and threatening human settlements.

Increased anthropogenic production of other gases such as methane (CH4) also contributes to atmospheric warming, but carbon dioxide has been a focus of concern because it is emitted in much greater volume.

No one knows how seriously human activity may be affecting the large and turbulent patterns of climate. Sometimes a very subtle event can have magnified repercussions in larger wind, precipitation, and pressure systems, disturbing major climate patterns for decades. In many cases the climate appears to have a self-stabilizing capacity—an ability to initiate internal reactions to a destabilizing event that return it to equilibrium. For example, extreme greenhouse heating should cause increased evaporation of water. Resulting clouds could block incoming sunlight, producing an overall cooling effect to counteract heating.

Furthermore, human influences work on climate within a context of continually changing natural conditions and events. On a geologic time scale, temperatures, precipitation, and ocean levels have fluctuated enormously. A long series of ice ages and warmer interglacial periods began 2.5 million years ago and may still be going on. The last glacial maximum, with low sea levels because of extreme ice volumes, ended only 18,000 years ago—an instant in the earth's climate history.

Natural fluctuations occur on a more human time scale, as well. A summer of extreme drought and high temperatures in the United States in 1988 brought threats of global warming to the public's attention, but the drought itself resulted from a temporary aberration in high altitude wind patterns that centered an unusually stable high pressure zone over the Midwest. This temporary departure from normal conditions was simply part of the continual fluctuation within the chaotic climate system. Terrestrial events, such as the 1991 eruption of Mount Pinatubo in the Philippines, also cause large, natural disturbances in climate. Dust from its eruption reached the upper atmosphere and was distributed around the globe, blocking enough incoming solar radiation to temporarily cool global temperatures by about 1.8°F (1°C).

No one can yet predict with precision how climate variables will respond to human activity. The earth's climate is so complex that human alterations to the atmosphere (such as those caused by carbon dioxide emission ) amount to an "experiment" having an unknown—and possibly life-threat ening—outcome.

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Climate

Climate

Latitude. The climate of any region is largely determined by the region's relation to the sun, which is called latitude, as well as its relation to water, winds, and elevation. Latitude is the most obvious and sometimes most deceptive determinant of climate. The southern extremes of Europe, such as Barcelona and Rome, are at a latitude equal to northern portions of the United States, such as New York. But the European climate of Madrid and Rome is much warmer than that of New York due to ocean currents and high-altitude winds known as the jetstreams. Most of Europe has a mild, temperate, and humid climate. Temperate-zone climates have a pronounced difference between winter and summer conditions. The most notable exception is the snow forest climate in some portions of Scandinavia.

Ocean and Wind Currents. Europe, the only continent with no desert region, is a peninsula jutting off the Eurasian landmass, and thus its climate is greatly influenced by the huge bodies of water to the west, north, and south. Water currents and winds coming off of the water have a great influence on European climate. The influence is especially evident in the northern plains where there are neither mountains nor highlands to interfere with wind patterns. The North Atlantic Drift is a broad ocean current, essentially an eastward extension of the North American Gulf Stream, of relatively warm water that hits the western shores of Europe. Prevailing westerly winds coming off of the Atlantic Ocean combine with the currents to create a temperate, humid, marine climate. Europe's mild, moist winters and cool, wet summers diminish as one travels east across the continent. The Atlantic Ocean heats up and cools off more slowly than the lands, and thus mild, moist climates along the coast differ from the cooler, drier winters and warmer, drier summers of the eastern portions of the Great European Plain. No north-south mountain barriers prevent warm Atlantic maritime air masses from moving east across Europe. As a result, climatic changes from the Atlantic to the Ural Mountains tend to be slow and gradual. Conversely, the absence of mountains also allows cold continental air to move westward in the winter, thereby creating long periods of winter weather across the Great European Plain.

Mediterranean Basin. The most dramatic variations in temperature are based not on the Atlantic but rather on proximity to the Mediterranean Sea, which borders Europe to the south. The climate of the Mediterranean Basin consists of dry, hot summers and moist, mild winters. A large high-pressure belt covers the Mediterranean Basin in the summer and creates droughtlike conditions. This subtropical high-pressure belt shifts slightly south in the winter, and the prevailing westerly winds bring enough humid air from the Atlantic Ocean to create wet, rainy winters. The northern edge of the Mediterranean Basin is clearly delineated by the various mountain ranges: Pyrenees, Appenines, Alps, Balkans, and the Caucasus. The mountains separate the northern humid climate from the dry subtropical climate of the Mediterranean Basin.

Six Climatic Zones. Europe can be divided into six broad climatic zones: the Mediterranean Basin (subtropical with fall and winter precipitation), Alpine Europe (climatic variation due to different elevations), Western-Northwestern Europe (mild winters and moderate to heavy precipitation), Central Europe (cold winters and summer rainfall), Eastern Europe (very cold, dry winters), and the Scandinavian Mountains (snowforests with heavy precipitation on the west). The Mediterranean Basin has a warm climate with dry summers. Climate is not uniform in the basin because of the various peninsulas and the diversity of the many smaller seas that make up the Mediterranean Sea. The Mediterranean Basin has less cloudiness than the rest of Europe in summer because the westerly winds move to its north. In the fall and winter the Mediterranean Sea is warmer than in the spring; hence, precipitation is higher in the fall and winter. The high elevation and diversity of altitudes create climatic variations in Alpine Europe. North of the Alps the rainiest season is the winter, whereas south of the mountains the rainiest season is fall. The mountains themselves have a fairly narrow high altitude area, and thus only a small region experiences the typical snow forest climate found in the Scandinavian Mountains. The maritime influence is most strongly noticeable in Western-Northwestern Europe, where southwesterly winds create a humid, temperate climate. Winters are mild and precipitation is moderate to heavy. The northern edges of this region are some of the cloudiest areas on earth. The maritime influence is less obvious as one travels east to Central Europe, but the absence of mountains makes it a transitional zone between the mild, wet northwest winters and the cold, dry Russian winters. Winter is the cloudiest season, but summer has the highest precipitation. Eastern Europe continues the gradual variation in climate. Late-summer rains lead to heavier precipitation in the fall than in the spring. The Scandinavian Mountains form a barrier that keeps much of the cloud cover and precipitation on the western side of Norway.

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Climate

Climate

Irrigation Agriculture. Egypt’s prosperity was founded upon irrigation agriculture. The surplus food that farmers produced supported the bureaucracy, priesthood, artists, and royal family. When the Nile flood brought new topsoil to the river valley and enough water to fill the irrigation canals, Egypt was stable and prosperous. When the Nile flood failed for long periods, the central government and all the activities that government supported collapsed.

Desert and River Valley. The Egyptian desert and river valley have two distinct climates. The desert’s climate depends on local rainfall, while the moisture level in the valley depends entirely on the water that the river brings from its source, the monsoon rains in Ethiopia.

Rainfall Levels. Local rainfall levels in the desert fluctuated within a narrow range during ancient Egyptian history. Yet, even small increases in rainfall made life possible in the oases of the Sahara. Bundles of leaves excavated in the oases indicate a slight increase in rainfall during the Old and Middle Kingdoms (circa 2675–1630 b.c.e.). These leaves have been radiocarbon dated to the Old Kingdom. Settlements in the Dakhla Oasis also flourished in the Old Kingdom. Relief sculpture from the reign of Nyuserre (Dynasty 5, circa 2455–2425 b.c.e.) depicted antelopes, ibex, gazelles, and ostriches

in the desert in a landscape of grass, shrubs, and small trees. A second period of wetter weather occurred circa 1210–1110 b.c.e. during Dynasties 19 and 20. Again there is some evidence of Ramesside settlement in the Sahara during this period. A tree root excavated in a desert wadi radiocarbon dated to 1150 b.c.e. Another increase in rainfall occurred in the period 65 b.c.e.–560 c.e. with a new flourishing of settlement in the oases.

Flood Levels. The Nile River brings water from its source in Ethiopia to the valley in Egypt. Thus, the amount of water in Egypt itself was unrelated to the amount of local rainfall. Local records for Nile flood levels are available beginning about 3000 b.c.e. The earliest records from the Nilometer on Roda Island in modern Cairo indicate that the flood decreased abruptly for nearly two hundred years until circa 2800 b.c.e. Karl W. Butzer has calculated a 30-percent decrease in the volume of water from the earlier period. Beginning in Dynasty 3 and lasting until Dynasty 5, there was an increase in Nile levels (circa 2675–2400 b.c.e.) that corresponds to the prosperity otherwise in evidence in Egypt during this period. About 2200 until 2000 b.c.e. the Nile flood failed, perhaps contributing to the collapse of the Old Kingdom. Both literary evidence and relief sculptures recorded social disorder and famine during this time period. Between 1840 and 1770 b.c.e. the Nile floods were high. This time period again corresponded with the prosperity enjoyed by Egypt during Dynasty 12. About 1300 b.c.e. the flood was high but began to fall after a period of good floods that correspond with Dynasty 18. Some areas in Lower Nubia received no floodwaters and were abandoned. By the twelfth century b.c.e. (circa 1170–1100 b.c.e.), low floods caused food shortages. Food prices rose sharply during this time period, according to records from Deir el Medina. Real instability in food sources probably led to the end of the New Kingdom. Stronger floods again occurred in the period from 950 to 700 b.c.e., guaranteeing the relative stability of Dynasties 21–24. Other periods of good floods during the first millennium b.c.e. were the fifth century and the first century, both periods of relative prosperity. Clearly, the evidence of good Nile floods correlates directly with evidence of prosperity and political stability in Egypt.

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Climate

Climate

Rainfall. China is impacted by cold, dry, continental air, but the warm, humid, oceanic air brings in most of its rain-fall. During the summer this sea air reaches further into the continent, sometimes as far as Mongolia. Naturally, a great deal more rain falls on South China than on North China. To the south, near the sources of cloud-carrying winds, rainfall is heavy because the coast is backed by mountains and this situation gives rise to moisture as it comes in from the ocean. Rain in the South averages almost sixty inches a year, and some coastal mountains receive as much as a hundred inches. This rainfall, together with run-off from the central Asian massif, supplies South China with plenty of water, which is used not only for irrigating rice fields but also for transportation. The network of navigable streams and canals, which covers most of the central and lower Yangzi basin and the less mountainous regions south of it, has been a great economic resource. In Central China the rain averages about forty inches, and in the Yangzi delta forty-five. In both South and Central China the summer humidity is high and temperatures range from sixty to eighty degrees Fahrenheit. In the North, far from the sources of moisture, rainfall declines and averages usually between twenty inches and thirty inches on the coast, and much less inland. The Northwest receives less than the twenty inches of minimum rainfall necessary for cultivating unirrigated lands and has little or no water transportation.

Boundary. An equally significant climatic boundary is the one between the farming areas of China Proper and the steppes to the north, which (unless some parts of the steppes are farmed) too arid for crops. This region also marks the linguistic and cultural border between the agricultural Chinese and the pastoral, Altaic-speaking nomads. During the imperial era (617-1644) the Chinese continued to rebuild and extend the Great Wall in order to defend themselves from their northern neighbors and distinguish themselves from other peoples.

Temperate Zone. China Proper lies almost entirely in the temperate zone. Seasonal differences are marked. In the spring and summer, moisture-laden winds carrying rain sweep northward from the ocean; in the autumn and winter the process is reversed. The air over the great northern and western land barriers cools more rapidly than that over the

tropical and subtropical seas to the south, and it moves southward, bringing cloudless skies and comfortable temperatures. Therefore, China Proper has most of its rain in the spring and summer.

Crops. Differences between the North and the South in rainfall and temperature help to distinguish North China from the Yangzi Valley and the south coast in terms of geo-graphical appearance and crops. During the imperial era in the South the plains and hills were more green than they are today, the growing season was from six to nine months in length, two or three crops a year were produced, and the widespread grain was rice. In the North the hills and plains were brown and dust blown during the winter, the growing season was only about four to six months, only one crop a year was planted, and the most prevalent crops were wheat, kaoliang (Chinese sorghum), millet, and beans. In the North trees did not grow easily, and the characteristic forests were of broad-leaved, deciduous varieties. Because of the climate and the nature of the soil only parts of North China were heavily timbered. In contrast, the longer growing season and the heavier rains of the South were favorable to trees. Vegetation was more abundant, and forests grew much faster.

Society. In the North little rainfall meant many famines, and the bitter winters adversely affected people’s health. The cold and the dust storms often kept people indoors in unhygienic conditions. Since fuel was expensive, houses remained badly heated. Heavy clothing was normal, and winter laundry and bathing were hard. Under such conditions disease thrived. Furthermore, the short growing season demanded intense activity for only several months in a year and enforced idleness during the rest of the year. Home industries occupied only part of the time during the slower seasons. In the Yangzi Valley and the South, on the other hand, the winter temperatures were milder. Dust storms did not occur, outdoor life was frequent, and bathing was possible.

Animals and Architecture. The climate contributed in part to the small number of domestic animals in the South. The humid, damp summers produced coarse grasses that proved difficult for cows, sheep, and horses to eat and digest. The climate also had an impact on Chinese architecture to a certain extent. During the imperial era, heavy summer rains forced the Chinese to use sound, heavy, tiled or thatched roofs to protect their houses.

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Climate is the weather experienced by a given location, averaged over several decades. A region's climate tells how hot or cold, wet or dry, windy or still, and cloudy or sunny it generally is. It also tells whether these conditions prevail year-round or if they change with the seasons.

Throughout history, climate has been an important factor in determining where groups of people choose to settle. While humans are resourceful enough to survive almost anywhere on the planet, most population centers are in areas where temperature and rainfall are adequate to sustain some form of agriculture. There are fewer settlements in regions of extreme dryness or cold, such as deserts or the arctic. Climate also influences how people live. It largely defines choices of architecture, clothes, food, occupation, and recreation.

Climate is determined not only by average weather conditions but also by seasonal changes in those conditions and weather extremes. Thus, for example, a climate can be described as hot and wet year-round, frigid and dry year-round, or warm and rainy in the summer and cold and dry in the winter. Certain parts of Asia and Africa have monsoon climates, meaning they are warm year-round and relatively dry for half the year yet experience heavy rainfalls over the other six months. In other regions of the world severe blizzards or hurricanes are irregular, yet not uncommon, occurrences.

The climates of the world are differentiated by many factors, including latitude (distance north or south of the equator), temperature (the degree of hotness or coldness of an environment), topography (the shape and height of land features), and distribution of land and sea. Climate is also defined by the plants that exist in a region. What's more, climate is not a fixed property of a region. The climate has changed considerably
over the 4.6 billion years since Earth was formed. It continues to change today, as it will in the future.

WORDS TO KNOW

acid precipitation:

rain and snow that are made more acidic by sulfuric and/or nitric acid in the air.

air pressure:

the pressure exerted by the weight of air over a given area of Earth's surface. Also called atmospheric pressure or barometric pressure.

the theory that average temperatures around the world have begun to rise, and will continue to rise, due to an increase of certain gases, called greenhouse gases, in the atmosphere. Also called enhanced greenhouse effect.

haze:

the uniform, milky-white appearance of the sky that results when humidity is high and there are a large number of particles in the air.

The two criteria that are most significant in defining climate type are temperature and precipitation (water that originates in the atmosphere and falls to the ground). These criteria, in turn, are influenced by a number of atmospheric, oceanographic, and topographic factors, such as: uneven heating of Earth by the Sun; distribution of land and water; ocean currents; wind circulation patterns; the locations of high- and low-pressure systems; mountain ranges; and altitude. Elements of secondary importance to defining climate are winds, humidity, air pressure, and sunshine versus cloud cover.

There are two temperature statistics that are relevant to climate: annual mean (average) temperature and annual temperature range. To calculate the annual mean temperature, you need a year's worth of daily mean temperatures. A daily mean is the average of a day's maximum and minimum temperatures. For instance, if a day's high temperature is 60°F (15°C) and the low is 30°F (−1°C), the average is 45°F (7°C). Then add up the average temperature for every day throughout a year and divide by 365 to get the annual mean. A given location's annual mean temperature can be compared to the world's average, 59°F (15°F), to determine the relative warmness or coldness of that location's climate.

The annual temperature range is found by subtracting the year's lowest mean monthly temperature from the year's highest mean monthly temperature. Thus, if a city averaged 80°F (27°C) in July, its hottest month, and 40°F (4°C) in January, its coldest month, it would have an annual temperature range of 40 degrees. Some places in the United States, such as San Francisco, California, have a temperature range as small as 10 degrees. The annual temperature range reveals whether or not a location experiences different seasons, which is just as important as the annual mean temperature in identifying climate type.

Standardized global temperature information has been corrected for the influences of topography by a process known as "adjustment to sea level." Temperature drops at a rate of 3.6°F for every 1,000 feet (300 meters) you ascend above sea level. Thus, when taking the temperature on a mountain at a height of 4,000 feet (1,200 meters), you would add 14.4 degrees from the reading on the thermometer to find the temperature adjusted to sea level.

If you plot out mean annual sea level temperatures throughout the world, you find bands (called isotherms) that roughly correspond to latitude. Isotherms are imaginary lines connecting areas of similar temperature. That is, temperatures are highest at the equator, decline with increasing latitudes, and are lowest at the poles. This is due to the uneven heating of Earth by the Sun. The Sun strikes Earth most directly near the equator and most indirectly (at the steepest angle) at the poles. The atmosphere, through the movement of global winds, strives to even out the distribution of heat but does not totally erase these differences.

The isotherms veer from their paths where landmasses meet the sea. This is due to the different heating and cooling behaviors of land and water (water takes longer to heat up and retains heat longer than land does). The bending of isotherms along the coasts is also caused by ocean currents, which carry warm water toward the poles and cold water toward
the equator. A third cause of this temperature differential at the coasts is upwelling, the rising up of cold waters from the depths of the ocean.

Two measures of precipitation are relevant to determining climate: total annual precipitation and a month-by-month breakdown of when precipitation occurs. The former statistic is an indicator of the overall wetness or dryness of a region, while the latter indicates whether or not a region experiences distinct rainy and dry seasons.

The amount of rainfall determines the type of plant and animal life a region can support. However, the distribution of precipitation throughout the year is also an important factor—particularly in relatively dry regions. For instance, if an area receives little total annual rainfall, but most comes during the summer months, that region may be suitable for agriculture. However, if the limited precipitation of a relatively dry region is spread more or less evenly throughout the year, it may not be able to sustain crops.

Wet and dry regions are scattered across the globe. There are some general trends, however, that correspond to global air circulation patterns. For instance, around the equator, where the trade winds (dominant surface winds that blow from east to west) converge and air rises, precipitation is relatively high. Precipitation is also high, about 60°, in the middle latitudes (the regions of Earth that lie between 30° and 60° latitude), where the westerlies meet the polar easterlies and air rises. Westerlies are global-scale surface winds that travel in the middle latitudes from southwest to northeast in the Northern Hemisphere (the half of Earth that lies north of the equator) and from northwest to southeast in the Southern Hemisphere (the half of Earth that lies south of the equator). Polar easterlies are cold global winds that travel across the polar regions, from the northeast to the southwest in the Northern Hemisphere and from the southeast to the northwest in the Southern Hemisphere.

In the subtropical regions around 30° latitude, where the trade winds meet the westerlies and air sinks, conditions are much drier. Most of the world's deserts are located there. The polar regions, where the polar easterlies converge and air falls, are also characterized by dryness. It should be noted that these latitudes are not fixed. As Earth makes its yearly revolution around the Sun, the amount of sunlight received at each

WORDS TO KNOW

isotherm:

an imaginary line connecting areas of similar temperature.

leeward:

the eastward side of the mountain, on which cold air descends, producing dry conditions.

middle latitudes:

the regions of the world that lie between the latitudes of 30° and 60°, north and south. Also called temperate regions.

monsoon:

rainy season that occurs during the summer on tropical continents, during which the land becomes warmer than the sea beside it.

obliquity:

the angle of the tilt of Earth's axis in relation to the plane of its orbit.

ocean currents:

the major routes through which ocean water is circulated around the globe.

orographic lifting:

the upward motion of warm air that occurs when a warm air mass travels up the side of a mountain.

the layer of Earth's atmosphere, between 25 and 40 miles (40 and 65 kilometers) above ground, that filters out the Sun's harmful rays. It consists of ozone, which is a form of oxygen that has three atoms per molecule.

paleoclimatologist:

a scientist who studies climates of the past.

permafrost:

a layer of subterranean soil that remains frozen year-round.

polar easterlies:

cold, global winds that travel across the polar regions, from the northeast to the southwest in the Northern Hemisphere and from the southeast to the northwest in the Southern Hemisphere.

a technique used to determine the age of rocks that contain radioactive elements, which works on the principle that radioactive nuclei emit high-energy particles over time.

rain shadow effect:

the uneven distribution of precipitation across a mountain, with most of the precipitation falling on the windward side and very little falling on the leeward side.

semipermanent highs and lows:

the four large pressure areas (two high-pressure and two low-pressure), situated throughout the Northern Hemisphere, that undergo slight shifts in position, and major changes in strength, throughout the year.

smog:

common name for photochemical smog, a layer of hazy, brown air pollution at Earth's surface comprised of surface ozone.

sunspots:

areas of magnetic disturbance on the surface of the Sun, sometimes referred to as "storms."

topography:

the shape and height of Earth's surface features.

trade winds:

dominant surface winds near the equator, generally blowing from east to west and toward the equator.

transpiration:

the process by which plants emit water through tiny pores in the underside of their leaves.

tropical storm:

a tropical cyclone weaker than a hurricane, with organized bands of rotating thunderstorms and maximum sustained winds of 39 to 73 mph (63 to 117 kph).

upwelling:

the rising up of cold waters from the depths of the ocean, replacing the warm surface water that has moved away horizontally.

virga:

rain that falls from clouds but evaporates in mid-air under conditions of very low humidity.

westerlies:

global-scale surface winds that travel from the southwest to the northeast in the Northern Hemisphere, and from the northwest to the southeast in the Southern Hemisphere, between about 30° and 60° latitude.

windward:

the westward side of a mountain, on which warm air ascends, forms clouds, and yields precipitation.

Precipitation is also connected with temperature in that warm air can hold more moisture than cold air. In the polar regions, the air holds so little moisture that precipitation is a rare event. This situation stands in contrast to the tropics, where warm air holds an abundance of water and rain is plentiful.

Another factor influencing precipitation is topography. As air travels over a mountain, orographic lifting, the upward lifting of warm air that occurs when a warm air mass travels up the side of a mountain, results in cloud formation and precipitation. Thus, the west (windward) side of a mountain range receives ample rainfall while the east (leeward) side stays relatively dry.

The positions of large-scale high and low pressure systems around the globe are a final factor influencing precipitation. These storm-producing
and storm-directing zones are influential in determining the westerly and easterly (horizontal) patterns of precipitation.

Although most people are familiar with certain climate types, such as tropical rain forest, desert, and tundra, no universally accepted, official system of climate designation exists. The first system to be used widely among scientific circles was the Köppen system, developed in 1918 by German meteorologist Wladimir Köppen (1846–1940). Köppen divided the world into five climate types, based primarily on precipitation and secondarily on temperature. They are humid tropical; arid (dry); humid subtropical; humid temperate; humid cold; and polar. Köppen then broke each of these climatic regions down into subclimates.

In the early 1930s, American climatologist C. Warren Thornthwaite (1899–1963) proposed another system, the Thornthwaite classification. He later revised the system in 1948 and again in 1955. Thornthwaite's system is based largely on precipitation levels but also takes into account the rate at which moisture returns to the atmosphere by evaporation (the process of water changing from a liquid to a gas) and transpiration (water lost through plant leaves and stems). He developed an index using the ratio of precipitation to evaporation and added a factor called potential evapotranspiration, the total amount of water that would be produced by a combination of evaporation and transpiration, given sufficient precipitation. He translated this formula into five different climate types named for their characteristic vegetation: rain forest, forest, grasslands, steppe, and desert.

Most of the classification schemes found in textbooks today, including the one presented here, are some combination of Köppen's and Thornthwaite's systems. In this chapter, the climates of the world will be described in the following general categories, some with subdivisions: humid tropical; dry; subtropical; temperate; subpolar; polar; and mountain.

Note: It would be helpful to have a world atlas beside you while reading this section to better understand the locations of the regions described.

Humid tropical climates, as the name implies, are warm and wet. The mean temperature for any month seldom falls below 64°F (18°C), so there is no winter. But there is plenty of rainfall in these climates. They receive on average about 60 inches (150 centimeters) of rain per year, which may be concentrated over a few months or spread throughout the entire year.

A key reference to: Climate types (hot/wet to cold/dry)

warm year-round with very rainy (flood-prone) summers and relatively dry winters. Covers much of southern and southeastern Asia, the Philippines, coastal regions of northern South America, and slices of central Africa.

world's driest regions, with less than 10 inches (25.4 centimeters) of rainfall annually; warm and cold varieties. Examples of warm deserts include: the Sahara (Africa), the Great Sandy (Australia), and the Mojave and Sonoran (southwestern United States). Examples of cold deserts include: the Atacama (Chile), the Great Basin (Nevada), and the Gobi (Asia).

Steppe:

semidry grasslands (receive less than 20 inches [50.8 centimeters] of rainfall annually) that occur in the rain shadow of a mountain range or on the edge of a desert. Covers portions of central Mexico, southern South America, northwestern Africa, west-central Asia, and the western United States.

long, cool summers and mild winters; coastal climate characterized by low clouds, fog, and drizzle for much of the year. Covers the coastal areas of the northwestern United States, the British Isles, New Zealand, western France, and southeastern South Africa.

four distinct seasons with warm, humid summers and cold, snowy winters. Covers the northeastern United States and southern Canada, and parts of central Europe, central Asia, southeastern Chile, southern Australia, and New Zealand.

bitterly cold winters and cool summers; for at least one month of the year the average temperature is above freezing. Covers the northern portions (except for the northernmost extremes) of North America, Europe, Asia, Iceland; and the coasts of Greenland and Antarctica.

Arctic:

barren lands of snow and ice that never thaw. Covers Ellesmere Island and the northern section of Baffin Island (Canada), and the interiors of Greenland and Antarctica.

Humid tropical climates are located in belts that are 20- to 40-degrees wide on either side of the equator. They cover the largest portion of Earth of any climate zone. They account for nearly 20 percent of the land and 43 percent of the oceans, totaling 36 percent of Earth's surface. This category is subdivided into three specific climate types: rain forest, monsoon, and savanna.

Rain forests are warm regions in which the rainy season usually lasts all year. Average rainfall for each month is over 2 inches (5 centimeters), and annual total rainfall usually exceeds 60 inches (150 centimeters). In some areas it may exceed 160 inches (400 centimeters) annually. On most days, heavy showers fall in the afternoon and skies clear by the evening.

Tropical rain forests exist at low elevations in regions very close to the equator. They cover the Amazon lowland of South America, portions of eastern Central America and the Caribbean Islands, the Congo River basin of Africa, the coastal area of western Africa, the East Indies from Sumatra to New Guinea, and a slice of Myanmar in southeastern Asia. Tropical rain forests also exist in isolated areas, such as the Atlantic coast of Brazil, the Pacific coast of Colombia, the Guiana highlands in northern South America, and eastern Madagascar.

Because of their proximity to the equator, rain forests experience about the same number of daylight hours year-round. Therefore, the annual temperature range is small, generally less than 6°F (3.3°C). The biggest change in temperature comes between day and night. The average daytime temperature is around 90°F (32°C), while the average nighttime temperature is about 72°F (22°C). The high humidity and cloud cover prevent the temperature of the rain forest from rising as high as it does on summer days in the subtropics and even in some parts of the temperate latitudes.

Tropical rain forests are also called tropical wet climates. They are noted for their abundant and varied plant and animal life. Whereas rain forests cover only 7 percent of the world's land, they contain 50 percent of all species of plants and animals. This total includes over 1.5 million species of plants and animals, the greatest biodiversity in the world.

Plants grow at all different heights throughout the rain forest, from the canopy (the uppermost spreading branchy layer of a forest) to the floor. The largest plants are the towering, broadleaf evergreens, such as mahogany and kapok trees. These trees block out most of the sunlight. Near the forest edge and in clearings where sunlight penetrates, vines and shrubs predominate. The undergrowth there is so thick that it's often impossible to walk through.

The great variety of plants provides sustenance for an equally impressive array of animals. In a rain forest, you will find animals occupying every possible niche: monkeys in the treetops; colorful birds such as toucans and parrots flying overhead; lizards scaling tree trunks; frogs, snakes, and insects on the ground; and termites tunneling underground. During the day, iguanas can be seen basking in the Sun while armadillos stay in their burrows to keep cool. Several species, such as fruit bats, moths, and caimans (cousins of the crocodile), become active at night.

In this type of tropical climate, the year is divided into dry and rainy seasons. While monsoon regions generally receive as much total annual rainfall as do rain forests, the precipitation is concentrated in the summer months. Typically, about 120 inches (300 centimeters) of rain falls during the rainy season, causing destructive flooding. Rainfall in the spring and fall is moderate while the months of December, January, and February are dry. The official designation of a monsoon climate is one in which monthly precipitation drops below about 2 inches (6 centimeters) for one or two months of the year.

The name monsoon comes from the Arabic word "mausim" for "season." A monsoon is actually a seasonal reversal of wind patterns. During winter, when the monsoon area is tilted away from the Sun, the winds that sweep across the region are primarily trade winds, blowing out toward the sea. These winds bring warm, dry air to the region. When spring arrives, the winds shift and carry moisture from the Indian Ocean over the land. At this time of year, the monsoon area is almost directly beneath the Sun. The land is heated intensely, which causes the surface air to rise. This air is replaced by the moist winds, which produce heavy rains that last throughout the summer months.

While monsoon climate is limited mostly to southeast Asia, a full 50 percent of the world's population lives in that area. The monsoon region encompasses most of India; the Indus Valley of Pakistan; portions of Nepal, Bhutan, Bangladesh, Myanmar, Malaysia, Vietnam, Thailand, Laos, and the Philippines; and the southeast tip of China. Some Atlantic coastal regions of northern South America also experience monsoon climate, as do parts of central Africa.

The total annual rainfall in monsoon regions is generally sufficient to sustain forests, even through the dry period. In some areas, these forests resemble rain forests, while in other areas the forests are less dense and

merge into scrublands. These lands are cultivated primarily with rice, a crop that thrives on flooding.

Animals that have evolved to survive in both wet and dry seasons live in monsoon regions. One example is the collared anteater, which eats ants in the wet season and termites in the dry season. Another example is the iguana, a reptile that can regulate temperature by changing its skin color and water loss by concentrating its urine.

A savanna (sometimes spelled "savannah") is a relatively flat land covered with coarse grasses that are several feet tall, and scattered, stunted deciduous trees such as baobabs and acacias. Like a monsoon climate, this climate is tropical, with wet and dry seasons. Yet the dry periods in savanna regions last longer than in monsoon regions, and annual precipitation is much lower.

Savannas are located farther from the equator than are rain forests and monsoon climates. While savannas are most common in south-central and eastern Africa, they also exist in the following locations: central and eastern India, western Central America, areas of Brazil north and south of the Amazon basin, southern Florida, parts of southeast Asia, and northern Australia.

Savannas usually receive between about 30 and 60 inches (75 and 150 centimeters) of rainfall annually, much less than rain forests and monsoon regions receive. In general, the farther a savanna is from the equator, the lower its annual rainfall. The amount of rainfall in savannas is not at all predictable, either from month to month or from year to year. In a single year, a devastating flood may be followed by a severe drought. A relatively wet year may be followed by one that is quite dry.

For at least five months out of each year, the savanna rainfall typically totals less than 2 inches (6 centimeters) per month. The dry season occurs during the winter months (December, January, and February), when high pressure systems are prominent in the region. In the summer, low-pressure systems predominate, and rainfall is abundant.

In addition to wet and dry seasons, a savanna has seasonal shifts in temperature. While the mean temperature stays above about 70°F (21°C) even in the coolest months, it hovers closer to 80°F (27°C) in the warmest months (March, April, and May). In the warm months, afternoon temperatures often range between 90 and 100°F (32 and 38°C).

African savannas are teeming with wildlife. Those grasslands are regions where lions, leopards, wild dogs, and other carnivores relentlessly

pursue speedy and well-camouflaged herbivores like antelopes, gazelles, and zebras.

There are two types of climate considered to be dry or arid: deserts and steppes. While deserts are the drier of the two, in neither climate does annual rainfall keep pace with the rate at which water is lost due to evaporation and transpiration. Dry climates are generally found in the region between 15° and 30° latitude (although sometimes at higher latitudes) and are sometimes bordered by mountain ranges. They cover about a quarter of the world's landmass. That total is greater than for any other climate type.

A climate's designation as dry depends on both its annual rainfall and its temperature. The mean temperature must be high enough to result in the evaporation of what little rainfall there is. To illustrate this point, consider the annual rainfall received by a hot New Mexico desert. If that same amount of precipitation were to fall on cold northern Canada, it would be enough to sustain a conifer forest.

Dry climates usually receive their sparse rainfall during the summer months, when temperatures are higher and evaporation occurs more readily. However, this rainfall is often quite irregular. One location may go two years without a drop of rain, then receive four inches in a single downpour.

Deserts are the world's driest regions. These true arid climates receive less than 10 inches (25 centimeters) of rainfall a year. Often, precipitation over these lands takes the form of virga, streamers of water that evaporate into the dry air before they even reach the ground.

Deserts come in hot and cool varieties. Hot deserts, which have no cool season, are the hottest places on Earth. Their mean temperature remains above 65°F (18°C) year-round. Cool deserts, on the other hand, have an annual mean temperature below 65°F and, for at least one month out of the year, a mean temperature below 45°F (7°C).

The daily temperature in both hot and cool deserts fluctuates greatly. The reason for this fluctuation is that little humidity is available to absorb incoming sunlight during the day and virtually no cloud cover to trap the heat escaping from the surface at night. In hot deserts, daytime temperatures commonly hover between 105 and 115°F (41 and 46°C) and sometimes exceed 120°F (49°C), while at night it cools off to around 75°F (24°C). Occasionally, during the winter months, the temperature at night in hot deserts drops below freezing. In cool deserts, afternoon temperatures in the summer often reach 105°F. However, it is not uncommon for nighttime temperatures in winter to dip below 30°F (™1°C).

The world's hot deserts are located in the subtropics between 15° and 30° latitude north and south, near the Tropic of Cancer and the Tropic of

Capricorn. Also known as the horse latitudes, these regions contain a high-pressure belt where air from the equatorial region descends, bringing warmth and clear skies. During the summer months, the Sun is directly overhead and produces the searing heat for which these deserts are famous. Some of the deserts in this region include the Sahara of Africa; the Great Sandy Desert of Australia; and the deserts of the North American southwest and Mexico, such as the Mojave and the Sonoran.

Cool deserts are located far inland and removed from any source of water in the middle latitudes, which lie between the latitudes of 30° and 60°. Many of these are in the rain shadow of tall mountain ranges, where precipitation is often less than 5 inches (13 centimeters) annually. Examples of deserts in this category include the Atacama Desert in Chile at the
base of the Andes Mountains; the Great Basin in Nevada at the base of the Sierra Nevada; the deserts of Nepal and northern India at the base of the Himalaya Mountains; and the Gobi in Asia.

Most deserts do have some vegetation although not a great deal. This vegetation takes the form of drought-resistant plants such as cacti, which store water in their stems and have waxy coverings that diminish transpiration, and scrubby plants like the creosote bush, which has an extensive root system. There are also a variety of wildflower species that pop up quickly following a rain only to drop their seeds and whither away once the soil dries.

Animals of the desert have adapted to the limited supply of water and food in many different ways. The camel, for instance, stores water and fat in its hump, reabsorbs moisture from exhaled air, and sweats only when its body reaches extremely high temperatures. The angulate tortoise of Africa stores water in its bladder, and reptiles retain moisture due to their thick, scaly skin. Other animals, such as snakes, spend the heat of the day in a burrow, while rabbits seek out the shade of a bush. Perhaps the ultimate example of an animal adapted to desert life is the kangaroo rat. Found in deserts of the American southwest, this rodent gets all the moisture it needs from solid food and can go its entire life without drinking a drop of water.

A steppe is a semiarid climate, meaning that it receives somewhat more rain than a desert. Steppes can be found along the edges of deserts and serve as transition zones between arid and moist areas. Annual precipitation in steppes generally measures between 8 and 20 inches (20 and 50 centimeters), all of which falls during a short wet season. This precipitation is enough to sustain short, coarse bunch grass called steppe (from which the climate gets its name), clumps of low bushes, sagebrush, and isolated trees.

Expansive steppes can be found in central Mexico, southern South America, northwest Africa, west central Asia, and surrounding the desert of central Australia. In the United States, regions with steppe climate are called prairies. These regions include the Great Plains (east of the Rocky Mountains), valleys on the northern boundaries of the Great Basin, and along the southern coasts of California.

An example of a city with a steppe climate is Denver, Colorado, which has an annual mean temperature of 50°F (10°C). Its monthly mean temperature fluctuates from 30°F (™1°C) in January to 75°F (24°C) in July, giving it an annual temperature range of 45°F (7°C).

Denver receives 15 inches (38 centimeters) of precipitation yearly, most of which falls between April and July.

The large animals living in steppes are primarily migratory, grazing mammals. Examples of these include deer and antelope in the United States; gazelles and ostriches in Africa; camels in Asia; and rodents that live on or under the ground, such as prairie dogs in the United States, gerbils in Africa, and hamsters in Asia.

Subtropical climates are those with distinct summer and winter seasons and sufficient rainfall to keep them from being classified as dry. At least eight months of the year they have mean temperatures above 50°F (10°C) and at least one month has a mean temperature below 65°F (18°C). An important criterion of a subtropical climate is that the winters are mild. The mean temperature of the coldest month always stays above 25°F (−4°C).

These climates are situated within a band running from about 25° to 40° north and south of the equator. This band begins at the edge of the tropics and extends through the warmer half of the middle latitudes, to the edge of the temperate zone. There are three distinct types of moist subtropical climates: humid subtropical, marine, and Mediterranean.

The most outstanding characteristic of a humid subtropical climate is its hot, muggy summer. The mean temperature during the summer months is about 75° to 80°F (24 to 27°C). The nighttime temperature seldom drops below 70°F (21°C) and during the day it often rises to more than 100°F (38°C). Intense heat waves are not uncommon and sometimes last for weeks. The relative humidity is high, even during the heat of the day, which makes for even more oppressive conditions. The dew point (the temperature at which water condenses out of the air) is often above 75°F.

Winters are generally mild and, on the lower-latitude end (closest to the equator) of a humid subtropical region, it is rare for temperatures to fall below freezing. On the higher-latitude (poleward) end, frost and snow are more common, but heavy snowstorms are still rare. Conditions change rapidly in winter. One day's warm, sunny weather can give way to the next day's cold rain.

Rainfall is plentiful throughout the year, totaling between 30 and 65 inches (about 75 and 165 centimeters) annually. Afternoon thunderstorms are common in the summer, as is precipitation from the edge of tropical storms. Much of the rain and snow in winter is associated with eastward-moving storm fronts.

This type of climate is found along the east coasts of most continents, between 25° and 37° latitude. It covers the southeastern United States, eastern China, southern Japan, southeastern South America (Uruguay, Paraguay, and southern Brazil), and the southeastern coasts of Africa and Australia. These locations are mostly on the western sides of subtropical high-pressure systems, placing them in the path of maritime tropical air that originates near the equator and is directed toward the poles.

Since the growing season lasts from eight months to a year, depending on location, this climate is well suited for many types of agriculture. The natural plant cover consists of thick deciduous and coniferous woodlands at the eastern edge of these zones, graduating to prairies on the western edge.

There is a minor subset of humid subtropical climate, examples of which include Portugal and the Canadian Pacific coast, that have mild, rainy winters, and cool, dry, short summers. During winter in these places, the ocean winds keep the air warmer than it is farther inland. The average temperature for the warmest months in these regions never exceeds 72°F (22°C) and three times as much rain falls during the winter as during the summer.

Marine climate, also known as Marine West Coast climate, features cool summers and mild winters, yet the summers are longer than they are in the humid subtropical climate of Pacific Canada. Precipitation is plentiful year-round in marine climates, and there is a relatively small annual temperature range. Some parts of the world with a marine climate are the west coasts of northern California, Washington, and Oregon; the British Isles; New Zealand; the west coast of France; and the southeast coast of South Africa.

This climate is characterized by the presence of low clouds, fog, and drizzle for much of the year, as well as regular rainfall and occasional snowfall. The temperatures are generally high enough that if snow does fall, it melts quickly. These conditions, plus relatively stable temperatures year-round, are largely the result of the prevailing westerly winds that bring ocean air into these regions.

On the U.S. northwest coast, rainfall is higher in the winter than in the summer. The reason for this pattern is the subtropical Pacific high-pressure system that directs storms away from the coast in the summer. It is common to find thick stands of coniferous and deciduous trees in marine climates.

The primary factor that distinguishes Mediterranean climates from other subtropical climates is that it has dry summers. In areas with a Mediterranean climate, there are four to six months each year with little or no rainfall. At least three times as much rain falls in the wettest (winter) month as in the driest (summer) month. Total rainfall for the year is between about 15 and 35 inches (38 and 89 centimeters).

The summer dryness is caused by subtropical high pressure systems located over the water, which produce sinking air and direct storms poleward, away from the coasts. The position of these highs shifts towards the equator in the summer, allowing mid-latitude storms (with plenty of rain) to enter the region.

The summer high temperatures are lowest at points nearest to the water's edge. This effect is caused by upwelling, the churning up of cold water from the depths of the ocean, which chills the coastal air. As one moves inland, the summers are hotter (and the winters are colder, too).

Bulgaria, the coasts of Turkey and Israel, and the Mediterranean coast of northern Africa. This region's reputation for warm, sunny summers attracts crowds of tourists from around the world each year.

Other parts of the world with a Mediterranean climate include the central and southern California coast, the southwestern tip of Africa, the southeastern and southwestern edges of Australia, and a small area on the west coast of central Chile.

The irregular rainfall and preponderance of summer wildfires means that the only types of vegetation that can survive in this climate are shrubs, dwarf trees, and grasses. The shrubs have extensive root systems, enabling them to gather what little moisture exists in the soil. The leaves of Mediterranean plants also have moisture-conserving designs. For instance, some leaves have hard, waxy surfaces; some are hairy; and others curl up when it's dry. The olive tree has gray-colored leaves, which reflect sunlight.

Mediterranean vegetation is also adapted to periodic wildfires. After a fire, many will resprout from their roots. Some plant species, such as the knobcone pine, actually depend on fire for their propagation. The cones of these trees open and release seeds only in the extreme heat of a fire. If there is any doubt about the ability of scrubland vegetation to withstand wildfires, one only has to witness the rapid growth that follows in the wake of the burn.

The native animal inhabitants of these lands are deer, rabbits, wolves, and numerous rodents such as squirrels, voles, and mice. In some regions, like California and southern Europe, cows, goats, sheep, and other domesticated grazing animals are now the dominant species.

Temperate climates are those that have four distinct seasons. The criterion that differentiates them from subtropical (the next warmest) climates is that for a least one month of the year the mean temperature in a temperate climate is below 25°F (−4°C). Temperate climates can be distinguished from subpolar (the next coldest) climates in that they have at least one month in the year in which the mean temperature is above 70°F (21°C).

All temperate areas have warm, humid summers and cold, snowy winters, although the average monthly temperatures and precipitation levels vary considerably from one place to another. Total annual precipitation ranges from 20 to 40 inches (50 to 100 centimeters) and is steady throughout the year.

Temperate climates are situated mostly between 40° to 60° latitude, primarily in the Northern Hemisphere. They cover the northeastern United States, southern Canada, and parts of central Europe and central Asia. In the Southern Hemisphere, where there is very little land mass south of the 40th parallel, temperate climates exist only in the southeastern portion of Chile, the southern tip of Australia, and New Zealand.

Toward the lower-latitude end of a zone of temperate climate, summers are hotter and more humid than they are in the higher latitudes. High temperatures in the summer are often over 90°F (32°C) in the lower latitudes, and there are five or six months of the year during which temperatures never dip below freezing. This means that the growing season is long enough to cultivate a variety of crops. In those same locations, the winter is cold and blustery with significant snowfall.

In the colder temperate areas, winter is harsher and longer, summer is milder and shorter, and spring and fall are merely quick transitions between winter and summer. Typically, there are only three to five months a year with no frost. During the summer, high temperatures rarely exceed 90°F (32°C) while in the winter the temperature may drop below −22°F (−30°C) and can remain below zero degrees for weeks on end.

The most common type of vegetation found in temperate areas is the deciduous forest. A deciduous forest contains trees such as oak, hickory, maple, beech, ash, and birch. In some areas there are also stands of conifers (evergreens) such as pine, spruce, and fir. In the autumn, a chemical transformation occurs within the leaves of deciduous trees, causing them to turn brilliant shades of red, orange, and yellow. In the spring, before the thick blanket of leaves has fully emerged, wildflowers dot the forest floor.

A great variety of birds, mammals, reptiles, and insects inhabit temperate areas. While many species of birds and some mammals migrate to warmer regions for the winter, other animals have developed methods of surviving through the cold months. Squirrels, for instance, store enough food in the summer and fall to last through the winter. Turtles find a deep, muddy burrow in which to hibernate. With many insect species, the adults die before the onset of winter, and their eggs hatch the following spring.

The subpolar climate zone is a cold, northern climate, found generally between 50 and 70° north latitude. It is dominated by a nearly continuous coniferous forest called the "taiga" or "boreal forest." Specifically, this region includes Alaska, central and northern Canada (except for the northernmost fringe), northern Europe (except for the north of Scandinavia), and northern Asia (except for the northern and southern fringes of Siberia). There is no subpolar land in the Southern
Hemisphere because, except for the very southernmost tip of South America, no land exists at these latitudes.

Subpolar climate is characterized by long, harsh winters and short, cool summers, with a wide range of temperature variation between the coldest and warmest months. The factor that distinguishes subpolar climates from neighboring, colder polar climates is that subpolar climates have one to three months in which the mean temperature is above 50°F (10°C). There is no limit, however, defining the cold end of the subpolar temperature range. In northern Siberia, for instance, the coldest monthly average temperature sometimes drops below −35°F (−37°C).

The cold air of subpolar regions retains little moisture. Thus there is little cloud formation and hence, light precipitation. Most subpolar lands receive less than 20 inches (50 centimeters) of precipitation annually. While snowfall is light, the air is so cold that once the snow falls, it remains for months. Because of the low temperatures, large tracts of subpolar land contain permafrost, a layer of subterranean soil that remains frozen year-round.

The short growing season rules out agriculture in the region. The main type of vegetation is conifer trees, such as fir, spruce, larch, and pine, plus a few varieties of deciduous trees, such as birch and alder. Conifers are well-suited to this environment. Their tall, narrow, conical shape

encourages the snow to slide off them, and their needles don't require much moisture to grow.

Large tracts of subpolar land are covered with highly acidic wetlands called bogs. Bogs are comprised primarily of sphagnum moss, hardy grasses, insectivorous plants, pondweeds, water lilies, and algae, as well as a smattering of flowering plants such as blueberry and azalea. When the moss dies, it accumulates as peat, which in some bogs forms a bouncy, carpetlike layer several feet thick.

The mammals of the boreal forest include the lynx and various members of the weasel family (wolverine, fisher, pine martin, mink, ermine, and sable). These animals prey on snowshoe hares, red squirrels, lemmings, and voles. Beaver are abundant, and their dam-building habit is an important part of forest succession. Large mammals of the boreal forest include moose and elk. Insect-eating birds such as wood warblers are migratory, but seed-eating birds (finches and sparrows, evening grosbeak, pine siskin, and red crossbill) and omnivores (ravens) may be year-round residents. All of these birds may move south if food is scarce. Mosquitoes are abundant in the spring.

The polar climate, which covers the extreme northern and southern portions of Earth, is the coldest in the world. The warmest month in these regions has an average temperature below 50°F (10°C). It is so cold near the poles because the sunlight strikes the surface there at a very steep angle and remains low in the sky even in the summer. This condition is in stark contrast to the world's hottest regions, where the Sun sometimes shines directly overhead.

The polar area extends, on its warmest boundary, to between 60° and 70° latitude. The coldest points are at 90° latitude, capping the North and South Poles. Lands with polar climates include: the northern reaches of North America, Europe, and Asia; all of Greenland; the northern half of Iceland; and Antarctica.

Polar regions receive less than 8 inches (20 centimeters) of precipitation annually, which is equivalent to that received by a desert. In a desert the air is warm enough for evaporation to exceed precipitation. In polar regions, however, the air is so cold that the meager rate of precipitation is still greater than the rate of evaporation, and the ground never dries out.

The polar climate is divided into two categories: tundra, which experiences a yearly thaw, and arctic, which remains a permanently frozen region.

Tundra comprises the majority of the polar land in the Northern Hemisphere, with the arctic being restricted to areas close to the North Pole. Due to the virtual absence of land between 50° and 70° latitude south, the only tundra in the Southern Hemisphere lies in narrow strips on the coasts of Antarctica.

In the tundra, the mean temperature of the warmest month is between 30 and 50°F (−1 and 10°C). While the tundra summers are cool, the winters are bitterly cold. An example of a tundra climate is Barter Island, Alaska, at 70° north latitude. The coldest month of the year at Barter Island is February, with an average monthly temperature of −20°F (−29°C). It has three months in which the average temperature is above freezing: in June the temperature reaches 35°F (2°C); and in July and August, 40°F (4°C). Throughout an average year, Barter Island receives about 7 inches (18 centimeters) of precipitation.

Beneath the tundra surface lies a layer of permafrost, frozen subterranean soil, hundreds of yards deep. In the summer the top few feet of soil thaw. Since the moisture can't penetrate the frozen ground below, the surface becomes muddy and swampy. The growing season is so short that only a limited number of small plant species dot the landscape. These include mosses, lichens, sedges, and dwarf trees. The trees are species of willow and birch; in warmer climates they grow several feet tall but in the tundra creep along the ground and are only a few inches tall.

What to watch

Moviegoing audiences were captivated in 2005 by the astonishing documentary March of the Penguins, a film about the annual mating and child-rearing cycle of Antarctica's emperor penguins. Aside from the incredible appeal of the penguins themselves, one of the most astonishing things about the film was the Antarctic landscape itself, so different from those humans inhabit as to seem almost like another planet. The climate plays a central role in the lives of these unusual birds, and director Luc Jacquet provides a look at the climate rarely before seen. March of the Penguins won an Academy Award for Best Documentary Feature, and is available on DVD.

A number of animal species have adapted to life in the tundra. Among them are waterfowl, most of which migrate south for the winter, and mammals with thick layers of insulating fat and fur. The ptarmigan and snowy owl are two birds that remain in the far north year-round. These birds each have a thick covering of feathers all over their body. The ptarmigan even has feathers on its feet. For protection from predators,

their feathers, like the fur of many animals, change color from brown in the summer to white in the winter.

Some small mammals, such as the arctic ground squirrel, survive the winter by hibernating. Heartier mammals, like the arctic fox, musk ox, caribou, and polar bear, remain active all winter.

The arctic is a barren land of snow and ice located around the poles in both hemispheres. There the average temperatures for every month are below freezing. In the Northern Hemisphere arctic lands include Ellesmere Island and the northern section of Baffin Island (these islands are part of the Canadian Archipelago) and the interior of Greenland, plus the seas between these lands. In the Southern Hemisphere it includes nearly the whole of Antarctica.

While temperatures vary greatly throughout the arctic, the coldest weather is found in Antarctica. Earth's lowest temperature, −128.6°F (−89°C), was recorded on July 26, 1983, at the Soviet research station Vostok in Antarctica. The station is located at 78° south latitude with an elevation of 11,400 feet (3,475 meters) above sea level. During Vostok's warmest month, December, the average temperature is −30°F (−34°C) and during its coldest month, August, the average temperature is −90°F (−68°C). The warmest temperature ever recorded there was −5.8°F (−21°C).

Antarctica is warmest on its outskirts and coldest in its interior. In the summer months, average temperatures range from 14 to 20°F (−10 to −7°C) near the seas and from about −5 to −20°F (−21 to −29°C) near the center of the continent. In the coldest months the outer areas have average temperatures about −20°F, while average temperatures in the interior are about −95°F (−71°C).

Conditions in the arctic region of the Northern Hemisphere are not much more hospitable. Average temperatures there are about −45°F (−43°C) in January, the coldest month, and about 10°F (−12°C) in July, the warmest month. The coldest temperature ever recorded in
Greenland was −87°F (−66°C) on January 9, 1954, at Northice, which is at a latitude of 78° north and an altitude of about 7,685 feet (2,340 meters). Farther south, at a latitude of 71° north, is Eismitte (which means "middle of the ice"). At this location, about 9,940 feet (3,030 meters) above sea level, the average temperature in February, the coldest month, is about −55°F (−48°C) and the average temperature in July, the warmest month, is about 10°F (−12°C).

Weather report: Emperor penguins

Emperor penguins are the only animals to breed in the winter on the coast of Antarctica. They can withstand colder temperatures than any other species. These birds maintain a body temperature of 100°F (38°C) even during the winter months, when the air temperature drops as low as −80°F (−62°C).

An emperor penguin is insulated by a thick layer of blubber. It also has waterproof feathers all over its body (even its bill and feet) except for its toes. The penguin is covered with more than seventy-seven feathers per square inch. A fluffy down at the base of each feather traps body heat.

The emperor penguin is the largest of any penguin species, standing 3 feet (just under 1 meter) tall and weighing about 88 pounds (40 kilograms). It also has the highest percentage of body fat of any penguin species and can survive up to four months without eating. The emperor penguin is so well insulated that it actually runs the risk of overheating during periods of physical activity!

Perhaps the only climatic factor more remarkable about the arctic than temperature is its patterns of sunlight. Because of the tilt of Earth's axis, day and night at the poles are each six months long. During the summer (March 21 to September 23 in the Northern Hemisphere and September 23 to March 21 in the Southern Hemisphere), the Sun never sets nor does it ever rise much above the horizon. The six months of winter, however, are
shrouded in complete darkness. The exception to this pattern is the six weeks of twilight, centered around the spring and fall equinoxes, the days marking the start of spring and fall that are the two days of the year in which day and night are most similar in length. During the six weeks of twilight (from February 7 to March 21 and from September 23 to November 6), the Sun is constantly just below the horizon.

The bitter arctic cold is accentuated by blustery winds, sometimes over 100 mph (160 kph). Snowfall, however, is very light. These regions receive less than 4 inches (10 centimeters) of precipitation annually, most of that during the summer months.

The surface of arctic lands is a permanent ice layer, extending to a depth of thousands of feet. Thus, there is no plant growth. There are, however, some robust animal species that inhabit the mildest fringes of this icy world. Principal among them are emperor penguins, insulated from the elements by a thick layer of fat and a dense coat of feathers. This animal is able to withstand conditions colder than any other and is the only animal species that breeds during the Antarctic winter. Fifteen whale species and six seal species, as well as numerous species of fish, populate the seas surrounding Antarctica. The only animals to inhabit the arctic interior are tiny insects such as springtails and microscopic invertebrate worms such as nematodes and rotifers.

There is no inclusive definition of a mountain climate, also called a highland climate. The reason is that a mountain climate encompasses a whole series of climate types that are found at various points on the way up a mountain. The primary reason that conditions change as you ascend a mountain is that temperature decreases with altitude. Air pressure, humidity, precipitation, and winds also change with altitude. The climates on a mountain form pockets within the general climate of the surrounding region.

Mountain climates can resemble climates found at higher latitudes. In general, each 1,000-foot (300-meter) increase in elevation will produce a change in climate similar to traveling about 185 miles (298 kilometers) towards one of Earth's poles. For every 1,000 feet (300 meters) gained in altitude, the temperature falls about 3.6°F (2.0°C). However, mountain climates are not exact matches to climate zones at higher latitudes because many factors are different. For example, the effects of global atmospheric circulation, extratropical cyclones, and other weather systems are not as significant in mountain climates.

A 13,000-foot-tall (3,960-meter-tall) mountain in the Sierra Nevada range, for example, encompasses five different climate types. A semiarid grassland surrounds the base of this mountain. Along the "foothills," the first 2,000 feet (610 meters), the grassland is gradually replaced by Mediterranean scrubland. From 3,000 to 6,000 feet (915 to 1,825 meters), there is the "montane zone," the climate resembles that of a northern temperate zone, with thick conifer forests. Above 6,000 feet, in the "subalpine zone," thinner stands of stunted conifers resemble a subpolar climate. This zone gives way to tundra at 9,000 feet (2,740 meters), in the "alpine zone." Finally, the mountain is capped with a permanent expanse of snow and ice.

Another notable feature of mountain climates is the uneven distribution of precipitation across a mountain or mountain range, known as rain shadow effect. Mountains have a relatively wet side, called the windward side, and a relatively dry side, called the leeward side. The windward side is the one encountered by approaching air masses. The windward side is on western slopes in the Northern Hemisphere and on eastern slopes in the Southern Hemisphere. The air rides upward along the mountainside, and as it rises, it cools. Once air reaches the dew point, condensation (the process by which water changes from gas to liquid) occurs and clouds form.

The animals that inhabit mountains are, for the most part, migratory. However, they do not follow typical north-south migratory paths. Rather they migrate up and down the mountain. They climb to higher elevations in the summer, when food is more abundant, and escape the harsh alpine winters by climbing down to warmer elevations. Among the wide variety of mountain animals are deer, bears, chipmunks, mountain goats, marmots, and many species of birds. Some animals are specially adapted to survive in
the oxygen-thin air found at high altitudes. One such animal is the guanaco, a relative of the llama, which has an unusually large heart and lungs.

Mountain climates are scattered throughout North America, South America, Africa, Europe, Asia, and the islands north of Australia. Examples of these climates include the Sierra Nevada and the Rocky Mountain ranges in the United States and Canada, the Sierra Madre Range in Mexico and Central America, the Andes in Chile and Peru, the Alps in Europe, and the Himalayas in southern Asia.

Throughout Earth's 4.6 billion year existence, the global climate has undergone continuous change. Most of what scientists know about the history of climate change is limited to the most recent 10 percent of Earth's lifetime. The farther back in time, the fewer clues there are about the climate. However, by using the clues that do exist and drawing conclusions based on more recent data, scientists have been able to construct a climatic picture spanning the entire history of Earth.

There have been times when Earth has been alternately warmer and colder than it is today. There have also been several periods during which significant portions of the planet's surface have been covered with ice, periods called ice ages. Each ice age has brought about the extinction of numerous species of plants and animals. The result is that the process by which plants and animals have evolved has not been a smooth one. Rather it has been halted many times and jump-started with each new warm period.

A key reference to: Climates of the United States

Within the continental United States, every climate is represented except the tropical, subpolar, and polar varieties. With 3,623,420 square miles (9,384,658 square kilometers) stretching from 49° latitude on the northernmost edge to 26° latitude on the southernmost edge, the United States contains perhaps a wider assortment of climates than any other nation. They are as follows:

Temperate: northeastern states, from New York to Maine, Michigan, Iowa, Wisconsin, Minnesota, and the eastern portion of the Dakotas.

Humid subtropical: southeastern states, from Pennsylvania to Missouri and every state to the south.

Semiarid: states in the rain shadow of the Rocky Mountains, including Nebraska south through Texas and portions of Montana, Idaho, Wyoming, Colorado, and New Mexico.

Marine: northwestern California and the western portions of Oregon and Washington.

Mediterranean: west coasts of central and southern California.

Mountain: portions of the west coast states (Washington through California) containing the Cascades and Sierra Nevadas, as well as portions of western and plains states containing the Rockies (Montana, Wyoming, Colorado, and New Mexico).

The history of global climate change can be broken down into four main periods: Precambrian, Paleozoic, Mesozoic, and Cenozoic.

The Precambrian era began with the formation of Earth about 4.6 billion years ago and ended 570 million years ago. Earth began its existence as a ball of molten liquid rock. Within about 900 million years its surface cooled and solidified. Volcanic eruptions spewed forth gases, such as nitrogen, argon, water vapor, sulfur dioxide, and carbon dioxide, trapped beneath the rocky surface. These gases rose and mingled with hydrogen gas, a byproduct of Earth's formation from solar gases, to create the early atmosphere. The water vapor condensed into rain, which fell to the surface forming vast oceans. The rain washed most of the sulfur dioxide and carbon dioxide out of the atmosphere, and oceans have covered much of Earth's surface since.

Soon after the formation of the early atmosphere, the first forms of life appeared. These were anaerobic (capable of living without oxygen) bacteria. Then, about 3.5 billion years ago, these bacteria evolved into blue-green algae, which had the ability to photosynthesize. Photosynthesis is a process driven by the heat of the Sun in which an organism combines water and carbon dioxide to form a simple sugar. Oxygen is given off as a byproduct.

Over time, oxygen accumulated in the atmosphere. By 350 million years ago, the concentration of oxygen in the atmosphere reached its present value of 21 percent. Oxygen molecules (O2) then combined with free oxygen atoms, which were formed when sunlight broke apart oxygen molecules, to form ozone (O3). The ozone formed a separate atmospheric layer and fulfills the important function of absorbing the Sun's dangerous ultraviolet rays. The oxygen in the atmosphere, plus the protection provided by the ozone layer (the layer of Earth's atmosphere, 25 to 40 miles above ground, that filters out the Sun's harmful rays), encouraged the burst of biological diversity that marked the end of the Precambrian era.

The Precambrian saw four ice ages. The first occurred somewhere between 2.7 billion and 2.3 billion years ago. Then Earth warmed up and was free of ice for almost a billion years. The second ice age took place between 950 and 890 million years ago; the third between 820 and 730 million years ago; and the fourth between 640 and 580 million years ago. In each case, some portion of Earth was iced over for about 100 million years.

The Paleozoic era began with a dramatic increase in the number of marine animal species. It lasted from 570 to 225 million years ago. The Paleozoic era was significantly warmer than the Precambrian
era, with the exception of two glacial periods. At the end of the first glacial period, around 400 million years ago, plants began to take hold on land.

The spread of plants across the land had a significant impact on climate. Primarily, it reduced the albedo (reflectivity) of Earth's surface by 10 to 15 percent. As a result, more sunlight was absorbed, which raised the planet's surface temperature.

During the last portion of this era, from 330 to 245 million years ago, temperatures fell again. About 250 million years ago, Earth's two super-continents, called Laurussia (containing Greenland, North America, Scandinavia, and most of Russia) and Gondwana (containing most of the rest of the landmass), joined together. The newly formed super-continent, Pangea, extended from the North Pole almost all the way to the South Pole.

Swiss American naturalist, educator, and fossil expert Jean Louis Rodolphe Agassiz (pronounced "A-guh-see") (1807–73) was a pioneer of the theory that ice ages have occurred in Earth's history. He was not the first, however, to come up with this idea. That honor belongs to Scottish geologist James Hutton (1726–97), whose ideas were shunned by other scientists. Agassiz, who was more persuasive in his arguments about ice ages, is generally credited with developing the theory.

Agassiz grew up in the Swiss Alps and spent much of his time exploring their great heights. He studied the huge boulders on the mountains and felt that they must have been

deposited there by glaciers. Thus, he began seeking evidence to prove that glaciers move.

A real breakthrough came in 1839 when Agassiz discovered that a house built upon a glacier had moved one mile during a ten-year period. This discovery prompted him to drive a line of stakes into the ice in order to trace their movement. In just a year he found that the stakes had moved into a U shape, with the stakes in the middle of the line showing the greatest change in position. Agassiz concluded that the glacier had moved the most slowly on the edges, where it was slowed by friction with the mountainside.

In 1840, Agassiz published his findings in a book entitled Studies on Glaciers. He wrote that within the past several thousand years, northern Europe had been covered by glaciers, in what he called a Great Ice Age. This notion was initially met with a great deal of skepticism in the scientific community.

In 1848, Agassiz became a professor of natural history at Harvard University. There he pursued many areas of study, principal among which was marine science. In 1859, he founded Harvard's Museum of Comparative Zoology. Meanwhile, Agassiz continued to search for evidence of ice ages in New England and around the Great Lakes. By the end of his lifetime, Agassiz's ice-age theory had gained a modest level of acceptance. It wasn't until the turn of the century that this concept became widely accepted.

The Mesozoic era, which is often known as the age of the dinosaurs, lasted from 225 to 65 million years ago. In the latter half of this
era, Pangea split into two continents, one containing North America and the other containing Eurasia (Europe and Asia combined). About 100 million years ago, these landmasses further subdivided, roughly into the present continents. However, the landmasses were still situated very close together.

Throughout the Mesozoic era, temperatures everywhere on Earth were, on average, 11 to 18°F warmer than they are today. They were also relatively uniform across the planet. This was most likely due to the efficient distribution of heat from the equator to the poles by ocean currents and global winds. Landmasses, even by the end of the Mesozoic era, were not as widely dispersed across the globe as they are today. Thus, ocean currents (the major routes through which ocean water is circulated around the globe) and winds had a relatively clear path between the equator and the poles.

The Mesozoic era experienced a number of temperature swings, culminating in a sudden, brief ice age. This coincided with the extinction
of about 70 percent of Earth's life-forms, including dinosaurs. According to one theory, an asteroid collided with Earth, creating a dust cloud that blocked out the Sun and brought about the ice age.

The Cenozoic era began 65 million years ago and continues to the present. Throughout this era, the continents have continued to drift, moving into their present configuration. The continental plates continue to move, and the process of super-continent formation and breakup will likely repeat several more times. This era is also characterized by the emergence of mammals, including humans, as the dominant animal group.

The Cenozoic era is divided into two parts: the pre-Holocene epoch, which stretches from the beginning of the Cenozoic until ten thousand years ago; and the Holocene epoch, which covers the most recent ten thousand years.

The pre-Holocene epoch, the early part of the Cenozoic era, was warmer than it is today, and there were no polar ice caps. Beginning about 55 million years ago, a long cooling trend began. This cooling occurred both gradually over time, and through a series of extremely cold periods. One of these cold spells took place about 50 million years ago, and another about 38 million years ago. The most recent was about 15

million years ago, the results of which can still be seen in the polar ice caps and the glaciers nestled in protected areas of tall mountains.

Geological Time Scale

Periods

Phanerozoic

Cenozoic

Neogene

24 million

Paleogene

65 million

Mesozoic

Cretaceous

144 million

Jurassic

206 million

Triassic

248 million

Paleozoic

Permian

290 million

Carboniferous

354 million

Devonian

417 million

Silurian

443 million

Ordovician

490 million

Cambrian

540 million

Precambrian

Proterozoic

2.5 billion

Archean

3.8 billion

Over the last 2.4 million years there have been two dozen ice ages—times when the global temperature has plummeted sharply downward. At seven different points over the last 1.6 million years, ice covered up to 32 percent of Earth's surface. Scientists estimate that throughout this era new ice ages have begun about every hundred thousand years and have been interspersed with warmer, interglacial periods, each lasting at least ten thousand years.

The most recent ice age peaked about between twenty thousand years ago, when there were glaciers up to 10,000 feet (3,000 meters) thick over most of North America, northern Europe, northern Asia, as well as the southern portions of South America, Australia, and Africa. The sea level fell and exposed large areas of land that are currently submerged, such as the Bering land bridge, which connected the eastern tip of Siberia with the western tip of Alaska.

This era was followed by a warm period, beginning fourteen thousand years ago. By eight thousand years ago—most of the ice had melted and between seven and five thousand years ago—the world was about 5°F warmer than it is today. The sea level rose and the current shape of continents emerged.

The Holocene epoch began approximately ten thousand years ago. Extensive climatic data exist for this time period. The history of human civilization begins during this post-ice age period, about six thousand years ago.

About five thousand years ago, when Earth was slightly warmer and wetter than it is today, agriculture was developed, and the earliest cities were established in Egypt and Mesopotamia (now part of Iraq). There was a relatively cool period that began about 900 bce and lasted until about 500 ce, which resulted in crop failures. There are also indications that beginning about 800 ce there was a prolonged drought, which in all likelihood contributed to the fall of the great Mayan civilization in Mexico and Central America.

In the Middle Ages (500 to 1500 ce), the global climate was similar to today's. During that time, the civilizations of Europe flourished, and the Vikings colonized Iceland and Greenland. However, a cold spell began at the end of the thirteenth century. Summer after summer was cold and wet, which caused famine throughout Europe.

Conditions were more moderate during the fifteenth century and then became colder again between about 1500 and 1850, a period referred to as the Little Ice Age. Rather than being continuously cold, the Little Ice Age consisted of a series of cold spells, each up to thirty years long, separated by warmer years. For the most part, this period was characterized by bitterly cold winters and cold, wet summers. The canals of Holland, as well as the Baltic Sea and the River Thames in England, were continuously covered with layers of ice several inches thick. Food became scarce throughout Europe, the mountain glaciers increased in size, and the colonists in Greenland and Iceland perished.

After the Little Ice Age, temperatures warmed. In the years since 1850, however, significant fluctuations have occurred in global temperatures. About a dozen cool periods have alternated with warmer periods. From 1900 to the present, there has been a 1°F increase in global temperature. Scientists now think that this increase constitutes part of a trend of human-influenced global warming and is not merely a natural part of Earth's continually changing climate.

There are various ways of looking at the Holocene epoch. Some consider it a warm period, since ice exists only at the polar regions, covering 10 percent of the planet. Others believe the world is still in the final stages of the most recent ice age. What most scientists do agree on, however, is that another ice age is in store for the future.

In an attempt to find some order in the series of climatic shifts that describes the history of Earth, scientists have sought to define a pattern of warm-cool cycles that repeat after a given period of time. They have, however, been largely unsuccessful. The proposed cycles either don't apply to all times in the past or don't hold true for all parts of the world.

One problem in determining patterns of climatic change is that many factors are involved. Some of those factors affect Earth's climate for millennia while others affect it only for decades. In addition, some factors, such as Earth's shifting orbit around the Sun, are predictable and regular, while others, such as collisions with large objects in space, are not.

Human activity constitutes a whole category of factors affecting climate change in the recent past and present. Deforestation, the burning of fossil fuels, and acid precipitation (rain and snow that are made more acidic when carbon, sulfur, and nitrogen oxides in the air dissolve into water) and smog (a layer of hazy, brown air pollution at Earth's surface) caused by industrial emissions are among the real or potential agents of climate change.

What follows is a discussion of a handful of forces that have affected the climate of Earth throughout its history: continental drift; shifts in Earth's orbit; volcanoes; asteroids and comets; and solar variability. Another phenomena that is currently being studied as a possible factor in long-term climate change is El Niño/Southern Oscillation (ENSO).

The continental drift theory was first suggested by German meteorologist Alfred Wegener in 1915. That theory states that all land on Earth was joined together in one huge continent 200 to 250 million years ago. Then, over the years, forces deep within Earth's core caused the land to break apart. Continents moved away from one another and drifted around the globe. This motion is predicted to continue indefinitely into the future.

Evidence of continental drift can be found in the fossils of dinosaurs and other mammals that migrated across once-connected landmasses and in the fossilized remains of tropical plants beneath polar ice sheets. Another piece of evidence that landmasses were once connected is that shapes of the continents fit together like pieces of a jigsaw puzzle. The Atlantic coastlines of Africa and South America are the most striking examples of this phenomenon. In addition, satellites are able to record the subtle, extremely slow movements of the continents today.

Continental drift is believed to affect both the climates of individual continents and the climate of the entire planet. The climates of the individual continents have been altered by their gradual, but radical, change in position around Earth. For instance, parts of Europe and Asia that once sat on the equator are now at high latitudes in the Northern Hemisphere. India moved to its current low-latitude Northern Hemisphere position from one deep in the Southern Hemisphere. Glaciers once covered parts of Africa, and Antarctica gradually slid from warmer latitudes to the South Pole.

On a global level, the position of landmasses affects how the heat from the Sun is distributed around Earth. When the continents were joined

together, ocean currents and global winds produced a different pattern of global heat distribution than they do at present. Global climatic conditions 200 to 250 million years ago were more uniform than they are today. As the continents separated and dispersed around the globe, greater extremes in climatic conditions began to appear around the world.

Another consequence of continental drift has been the formation of mountain ranges. When landmasses come together, the land is forced upward. Examples of mountain ranges formed in this way are the Rocky Mountains, the Andes, the Tibetan Plateau, and the Himalayas. Mountain ranges affect temperatures, winds, and rainfall over limited areas. Very tall ranges, particularly those with north-south configurations, can influence air circulation patterns over very large areas.

The Tibetan Plateau is a prime example of this phenomenon. Formed 50 million years ago by the collision of the Indian and Asian continents,
this plateau is one of the world's tallest and widest mountain ranges. It affects wind patterns across the entire Northern Hemisphere. Fossil records show that almost immediately following the formation of the Tibetan Plateau, the climate of the Northern Hemisphere cooled and large glaciers formed. The presence of glaciers led to further cooling. Snow accumulates on the ice, which reflects sunlight rather than absorbing it.

Another aspect of continental drift that affects global climate is the distribution of landmasses at various latitudes. For instance, as land moved away from the tropics and toward the poles, tropical oceans replaced the land. These bodies of water absorbed huge amounts of incoming heat, which led to global cooling. Also, the movement of continents into polar regions provided a surface on which ice layers could accumulate.

In the 1930s, the Yugoslavian astronomer Milutin Milankovitch (pronounced "muh-LAN-kuh-vich") (1879–1958) proposed a theory to explain changes in Earth's climate. He listed three factors that could affect the planet's climate: the shape of Earth's orbit, and the angle and direction of Earth's axis.

The shape of Earth's orbit around the Sun changes over long periods of time. At times, the orbit is nearly a perfect circle. At other times, it has a more elliptical shape (like an egg). The alternating change in the shape of Earth's orbit is called eccentricity. The eccentricity of Earth's orbit occurs in a regular cycle—from circular to elliptical to circular again—that takes about one hundred thousand years.

When the orbit is circular, there is less variation in the levels of solar energy received by Earth throughout the year than when it's elliptical. At present, the orbit is in a stage of low eccentricity, meaning that it is nearly circular. Earth receives only 7 percent more solar energy in January, when Earth is closest to the Sun, than July, when it is farthest from the Sun. In contrast, when the orbit is highly eccentric, the solar energy received at points closest to and farthest from the Sun will differ by up to 20 percent. In addition, a more elliptical orbit means the summers and winters are longer, and springs and falls are shorter.

The second orbital factor that affects climate is the wobbling of Earth about its axis of rotation. Earth's axis is tilted in relation to the plane of its orbit around the Sun. This effect is what causes the Northern and Southern Hemispheres to each receive different amounts of sunlight throughout the year.

Earth spins like a top in slow motion, so that its axis traces the path of a cone. It wobbles its way through one complete revolution every

twenty-six thousand years, a phenomenon called the precession of the equinoxes. This means that every thirteen thousand years the seasons are gradually reversed. Eventually, unless the calendar is adjusted, the Northern Hemisphere will experience winter in July and the Southern Hemisphere, in January. Precession also influences Earth's distance from the Sun at different seasons. When it's winter in the Northern Hemisphere in July, Earth will also be at its closest point to the Sun in that month.

The third variable affecting Earth's climate is the angle of the tilt of Earth's axis compared to the plane of its orbit. This angle is called obliquity. Over the course of forty-one thousand years, this angle fluctuates between 22 degrees and 24.5 degrees. It is currently 23.5 degrees. When the angle is smaller, sunlight strikes various points on Earth more
evenly, and the seasonal differences are smaller. That is, winters are milder and summers are cooler. Yet when the angle is larger, there is a greater variation in the level of solar radiation received across Earth, and seasons are more pronounced.

The smaller angle of tilt tends to favor the formation of glaciers in polar regions. The reason for this effect is that when winters are warmer, the air holds more water and snowfall is heavier. That snow then has a greater probability of remaining on the ground during the cool summer.

Evidence has been found in deep ocean sediments that strongly support Milankovitch's theory. By analyzing the chemical composition of these sediments, scientists have deduced that glaciers have advanced and retreated in roughly 100,000 year cycles over the last 800,000 years. Within those cycles, glacier formation occurs in secondary cycles, reaching peaks every 26,000 and 41,000 years. These time intervals correspond to the cycles of the three types of orbital variation described here.

In the early stages of Earth's history, thousands of volcanoes dotted the planet's surface. These volcanoes underwent frequent, large eruptions that had a significant impact on the climate. In addition to releasing gases that rose up and formed Earth's atmosphere, these eruptions sometimes spewed out ash and dust so thick that they very nearly blocked out the Sun. Volcanic eruptions have probably been the catalysts for some periods of glaciation.

While volcanic eruptions still occur today, they are far fewer in number and intensity than they once were. A very large volcanic eruption today affects global climate only for a few years. For example, in 1815 the Indonesian volcano Mount Tambora erupted. Dust from the eruption was carried around the world by upper-air winds. In conjunction with smaller eruptions of other volcanoes over the preceding four years, the Tambora event led to a decrease in global temperature. For example, a severe cold spell in 1816 earned that year the nickname "the year without a summer."

The eruptions with the gravest climatic consequences are those rich in sulfur gases. Even after the ash and dust clears from the atmosphere, sulfur oxides continue to react with water vapor to produce sulfuric acid particles. These particles collect and form a heavy layer of haze. This layer can persist in the upper atmosphere for years, reflecting a portion of the incoming solar radiation. The result is a global decrease in temperature.

Throughout Earth's history, there have been five very abrupt and dramatic changes in global climate, occurring 500 million years ago, 430 million years ago, 225 million years ago, 190 million years ago, and 65 million years ago. One possible explanation for these changes is that large, rocky bodies from space, such as an asteroid or comet (together known as bolides) crashed into Earth. Many scientists believe that the extinction of the dinosaurs, which came about 65 million years ago, was caused by a collision with an object from space.

It has been calculated that the impact of an asteroid at least 6 miles (10 kilometers) across, traveling at a speed of at least 12 miles (20 kilometers) per second, would produce a crater about 95 miles (150 kilometers) in diameter. It would release energy equivalent to that of four billion atomic bombs similar to those dropped on Hiroshima, Japan, heating the atmosphere to temperatures of 3,600 to 5,400°F (about 2,000 to 3,000°C). Another result of this energy release would be the production of huge concentrations of nitric and nitrous acids. These acids would react with and destroy the ozone layer. They would fall to the ground as highly acidic rain, destroying plants and animals.

If the object from space were to fall on land, a thick dust cloud would rise up and potentially block out all sunlight for several months.

Following an early wave of wildfires caused by the high temperatures, any surviving plants and animals would be killed off during a long, dark period of cold weather during which virtually all sunlight would be blocked from Earth by a thick dust cloud.

It is more likely that a bolide would fall into the ocean, since oceans cover almost three-quarters of Earth's surface. In this case, the bolide impact would stir up carbonate-rich sediments and produce vast quantities of carbon dioxide. An increase of carbon dioxide in the air serves to trap reradiated infrared radiation from Earth's surface, leading to an increased greenhouse effect, which is the warming of Earth due to the presence of gases that trap heat.

The possibility of such a collision happening within our lifetime is remote. However, it has become more of a concern following the July 1994 crash of fragments of Comet Shoemaker-Levy into Jupiter. These collisions, which occurred over several days, caused disturbances on an Earth-sized area of the giant gaseous planet's atmosphere. Had the same impact occurred on Earth, the results would probably have been disastrous.

It has long been established that the amount of energy emitted by the Sun varies slightly over the years. In recent years, scientists have made correlations between changes in cycles of solar output and particular weather patterns. While there have been several theories linking solar variation to long-term climate change, many more years of data collection will be necessary before such links can be proven. However, the evidence collected thus far presents a compelling case for the link between solar variability and climate change—at least climate change on the scale of decades.

The variation in solar output is primarily based on cycles of sunspot activity. Sunspots are dark areas of magnetic disturbance on the Sun's surface. It has been shown that when the number and size of sunspots is at a maximum, which occurs roughly every eleven years, the Sun's energy output is highest. This heightened solar output is due to an increase in bright areas, called faculae, which form around the sunspots.

Measurements taken on board satellites by special instruments called radiometers have shown that 0.1 percent, and possibly up to 0.4 percent, more solar energy reaches Earth during a sunspot maximum than during a sunspot minimum. (A sunspot maximum is a period of the greatest number of sunspots; a sunspot minimum is a period in which there are the smallest number of sunspots.) The length of sunspot cycles varies over time, from seven to seventeen years. A growing body of evidence supports a link between the length of sunspot cycles and temperature patterns around the world. It has been shown over the last century that global temperatures, in general, are higher during shorter sunspot cycles than during longer sunspot cycles. In addition, a reduction in the amount of sea ice around Iceland, another sign of warming, has been noted during shorter sunspot cycles.

Another piece of evidence linking sunspots and global temperatures is that the period of lowest sunspot activity in several centuries coincided with the coolest period in several centuries. Between 1645 and 1715, the stretch of years known as the Maunder minimum (named for British solar astronomer E. W. Maunder who discovered it in the late 1880s), sunspot activity was at a very low level. It is even possible there were no sunspots at all during this period. The period between 1645 to 1716 was also the coldest part of the Little Ice Age.

A paleoclimatologist, a scientist who studies climates of the past, uses a wide variety of methods. The first step in learning about climates of the past is to discover objects that were formed long ago and to put an accurate date on those objects. Then the challenge is to extract information from the objects that describe the climate at that time.

Many types of materials, both on Earth's surface and embedded far underground, that have been preserved over thousands and millions of years provide clues about Earth's past. Paleoclimatologists use these materials in a variety of ways.

The oldest rocks on Earth are about 3.9 billion years old. These rocks are the only objects that still exist from the earliest period in the Earth's history, the Precambrian era. Thus, scientists must rely entirely on rocks to learn about the climate of that time.

To determine the age of rocks, paleoclimatologists use a technique developed in the late 1800s called radioactive dating. This can be used for
rocks that contain radioactive elements, such as uranium, radium, and potassium. Radioactive nuclei exist in an unstable configuration and emit high-energy particles (alpha particles or positrons) over time, to achieve greater stability. When the parent nuclei shed alpha particles, or decay, they transform into daughter nuclei (in the case of a uranium parent, the daughter is lead).

The age of a sample is determined by comparing the percentage of parent nuclei to daughter nuclei. Since scientists know the rate at which radioactive elements decay, they can predict how long ago the sample was entirely made of parent atoms—in other words, when the sample was formed.

A similar technique is called carbon dating. This involves the analysis of radioactive carbon, which is produced in small amounts continuously over time in the atmosphere by cosmic rays (invisible, high-energy particles that bombard Earth from space). Radioactive carbon, like normal carbon, becomes assimilated into green plants through photosynthesis. Radioactive carbon is unstable because its nuclei contain too many neutrons. To achieve stability, the nucleus transmutes into nitrogen. Given the rate of decay of radioactive carbon it's possible to determine the age of a given sample.

Once scientists have established the age of a sample, they can study it for clues about the climate at that time. First, the shape of a rock tells them about the medium in which it once existed. For example, rocks with rounded surfaces probably once existed in a body of water, and rocks with eroded (worn-away) surfaces were probably once covered by glaciers.

To take this a step further, if rounded rocks from the same time period are found all over Earth, it can be assumed that the average temperature at that time was above freezing. The presence of surface water also indicates the existence of some form of atmosphere, without which water would quickly evaporate away. By the same token, if eroded rocks of the same age are found at far-flung locations, this signals that an ice age was in progress.

Fossils of primitive organisms begin showing up in rocks dated about 3.8 billion years ago. Studying fossils provides information not only about the progression of life-forms at different time periods but also about the climate. For instance, rocks formed during cooler times, such as ice ages, have few or no fossils embedded within them. In contrast, rocks that were formed during warmer times contain fossils in far greater numbers.

A particularly valuable source of evidence of climate change is rock formations made from layers of particles, deposited incrementally and hardened over time. An analysis of samples of each layer of a rock formation provides information about Earth's climate at that point in
history. Of particular interest are the fossils within each layer, which constitute a timeline of the emergence of various species.

The presence of marine animals in a given layer indicates that at that time the region was covered with water. By studying the chemical composition of fossilized shells, the primary ingredient of which is calcium carbonate, it's even possible to determine the relative warmth of the water. Oxygen in calcium carbonate exists in two forms: oxygen-16, which is by far the most plentiful, and oxygen-18 (the number refers to how many neutrons the atom possesses). It has been found that during periods of glaciation, the concentration of oxygen-18 increases in the oceans. Thus, the ratio of oxygen-18 to oxygen-16 in fossilized shells gives an approximation of the water temperature.

Ice cores drilled into the 2-mile-thick polar ice caps provide unique insights into climates of the past. The ice has been accumulating in layers, one year at a time, for thousands of years. Though individual layers of ice can be counted for the last 50,000 years in Greenland, patterns can be discerned from this ice that yield information as far back as 250,000 years. In Antarctica precipitation is so meager that annual layers of ice cannot be distinguished from one another. However, information contained in the thick layer describes conditions for as far back as 500,000 years ago.

The layers of ice contain many types of climatic evidence, all perfectly preserved. For instance, the thickness of a layer tells how much precipitation fell during a given year. That, in turn, yields information about temperature, since greater levels of precipitation fall during warmer times.

Chemicals detected within the ice are also clues to the air temperature at the time of the precipitation The approximate temperature during a particular period can be determined by comparing ratios of oxygen-16 and oxygen-18 present in a given medium.

By determining the nature of the dust contained within ice cores from the North Pole, it's possible to determine the origin of that dust. That, in turn, provides information about the wind patterns at that time. This type of evidence is not useful in South Pole ice cores, since Antarctica is surrounded by oceans, and is a great distance from all other landmasses.

In addition, air bubbles can be analyzed for the composition of the atmosphere. The timing of volcanic eruptions can be inferred from the presence of high levels of acidity in the ice. When volcanoes erupt, they emit dust and gases that are highly acidic.

The sediments collected from the bottoms of oceans and lakes contain information about the global climate dating back millions of years. The sediment accumulates in

layers, much as rock formations and ice do. The age of sediment layers is determined using chemical analysis.

Embedded within the layers of sediment are fossils of tiny marine organisms that have evolved and become extinct over time. Each species was adapted only to a narrow range of water temperatures. Therefore, the presence of a species in a given layer of sediment reveals the ocean temperature at that time.

Pollen has also settled in layers on ocean and lake floors. Pollen provides clues to past conditions because every plant species requires a particular set of conditions for survival. The first step in pollen analysis is to identify the pollen species. The next step is to determine its age by finding the age of the surrounding sediment. Studying the times in which particular plant species inhabited a given location teaches scientists about that location's climatic history.

For example, a recent study of a bog in northern Minnesota turned up pollen of fourteen plant types dating back eleven thousand years. In the oldest layer, the greatest pollen concentration was spruce. Since spruce trees inhabit cold habitats, it could be inferred that at that time conditions were cold. The next layer yielded primarily pine pollen. Since pine trees grow in warmer regions than do spruce, conditions must have been warmer then. In the next layer, dated eighty-five hundred years ago, oak pollen was widespread. Oak grows in drier conditions than does either spruce or pine, which means that conditions must have been drier at that time. By examining the pollen within each layer, it was possible to construct a climatological history of the bog.

Dendrochronology, the study of the annual growth of rings of trees, is another important component of paleoclimatology. Trees are the oldest living entities on Earth. Some bristlecone pines that are still alive today date back to the time of Julius Caesar.

As trees grow, they add new cells to the center of their trunk each year. These cells force the previous years' growth outward, forming concentric rings with the oldest ring on the outer edge. A tree's woody material acts like a library of climatic data, creating a record for each year of its lifetime. This information can be found in living trees, dead (but not rotten) trees, and tree stumps.

In order to assess the overall conditions of a given year, the width of that year's growth ring is measured. In warm, wet years, trees grow more (and have wider growth rings) than in cool, dry years. Hence, the difference in the width of growth rings is an indicator of climate.

In order to separate the effect of temperature from that of precipitation, it is necessary to examine trees growing at the edge of their temperature or rainfall range. An example of this is the fir trees growing at the edge of the subpolar zone in Canada. Since temperatures are quite cold every year, an increase in growth from one year to the next would be due almost entirely to precipitation. To study temperature, consider the case of Joshua trees in the heart of a North American desert. There rainfall is continuously very light, so variations in the width of tree rings would be caused by temperature changes.

Experiment: Create a weather "log"

You, too, can be a dendrochronologist. All you have to do is locate a slice of a tree trunk. You can use the stump of a recently cut tree or seek out a log that hasn't yet been split, from a firewood seller. Just be certain that you know the year in which the tree was cut and, if you're using a log, find out where it's from.

First count the rings on the face of the log to determine the age of the tree (a tree adds a new ring of growth each year, with the most recent year's growth in the center.) Then study the width of each ring. Remember that trees grow more in warmer and wetter years, resulting in wider growth rings.

Now you're ready to construct a basic climatic history for the area in which the tree grew. On a piece of paper, create two columns: one for the year and the other for the conditions. For years in which the ring is skinny, write "cold/dry." For years in which the ring is wide, write "warm/wet."

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Climate

Climate

Global climate is the long-term pattern of temperature and precipitation on Earth's surface. Heat and water are unevenly distributed around the globe, and Earth has many climate zones (areas with a characteristic climate) and subclimates (areas with unique climate features within a climate zone) with unique patterns of temperature, rainfall, winds, and ocean currents (the circulation of ocean waters that produce a steady flow of water in a prevailing direction). Climate zones support communities of plants and animals (ecosystems) that have adapted to thrive there. The term climate refers to temperature and moisture patterns that characterize a large region over tens, hundreds, or even thousands of years. Local changes that last days, weeks, or seasons, like storms and droughts, are called weather.

Regulating sunlight: the ozone and greenhouse layers

Energy from the Sun drives Earth's climate and biology. Sunlight heats the surface and nourishes plants that, in turn, feed animals. Heat drives ocean currents, winds, and the hydrologic cycle (the circulation of water between the land, oceans, and the layer of air surrounding Earth, called the atmosphere). Layers of gas in the atmosphere regulate incoming solar energy and maintain the planet's average temperature at about 60°F (16°C). The gaseous layers keep Earth within the temperature range where life-sustaining oceans are liquid and life flourishes.

Unfiltered sunlight is too strong for organisms; it damages plants and burns animal tissues. A layer of ozone gas in the outer atmosphere acts as a shield that protects Earth from the
Sun's most dangerous rays—ultraviolet radiation. Ozone absorbs most of the Sun's ultraviolet rays. The filtered sunlight that reaches the surface has the correct intensity to set off a process in green plants in which they produce their own food. When sunlight strikes objects on Earth's surface, they warm up and radiate heat back toward outer space. Gases like carbon dioxide and water vapor keep Earth warm by trapping heat in the lower atmosphere. They are called greenhouse gases because they warm Earth's surface the same way a greenhouse stays warm in the winter.

How climate works

Sunlight falls unevenly on Earth's surface. The Sun's rays are more direct at the equator and less direct at the North and South Poles, causing surface temperature to decrease the farther away the area is from the equator. Temperature also decreases with altitude (elevation), making it very cold on high mountain peaks. This uneven heating creates heat-driven flows in the oceans (currents) and atmosphere (winds). Circular patterns of rising warm air and sinking cool air in the atmosphere (called Hadley cells) control the distribution of rainfall. Six Hadley cells, three in each hemisphere (half of the Earth), create wind belts (consistent winds in a prevailing direction) and climate zones.

To illustrate, follow a volume of air as it completes a trip around the Hadley cell north of the equator: Intense sunlight heats ocean water in the tropical zone around the equator. The air warms, and gains moisture from the warm water below it. It rises and flows north of the equator, cooling as it moves. Because cool air holds less moisture than warm air, the water vapor condenses (changes to liquid from a gas) into clouds and falls as heavy rain in the tropics. Once dry, the air flows north and sinks over one of the hot, dry deserts north of the tropics like the Sahara. The dry air then flows back along the surface toward the equator. Earth's eastward rotation causes the returning winds to bend toward the west; they are the strong, steady Trade Winds that blow on either side of the equator.

Climate zones

Earth's climate zones are defined by their average yearly rainfall (or snowfall) and temperature. In general, they are alternating, east-west oriented, wet and dry zones under the rising and falling Hadley cells. If Earth were a simple, water-covered ball,
without complicating factors like continents, high mountain ranges, and ocean currents, there would be five climate zones in each hemisphere: tropical, arid, temperate, cold, and polar.

Tropical (hot, wet): Lush, biologically-diverse rainforests thrive in the tropical zone at the equator. The jungles of central Africa, the Amazon basin in South America, and south Pacific Islands like Borneo lie in the tropical zone.

Subtropical arid and semi-arid (hot, dry): Earth's great deserts lie in arid zones north (Saharan, Arabian) and south (Kalahari, Australian Outback) of the equator. Dry, semi-arid (mostly arid) grasslands form the bordering lands around the subtropical deserts. The African savannah, Asian steppe, and Great Plains of North America are semi-arid grasslands that support large mammals like elephants, horses, and buffalo.

Temperate (mild temperatures, moderate rainfall): A large percentage of Earth's population lives in mild and temperate regions of North and South America, Europe, and Asia. These climates often have warm, dry summers and cool, wet winters. Coastal regions are usually wetter and have less extreme temperature variations than inland temperate regions.

Cold (cold, moderate rainfall): The cold, snowy, northern forests of North America, Scandinavia, and Asia are called the boreal zone. The treeless plain of the sub-arctic between the boreal forest and the polar ice cap (the thick covering of permanent ice and snow at the North and South Poles) is called tundra.

Polar (very cold, very dry): The North and South Poles are cold, dry deserts. The polar ice caps have formed from the accumulation of light snows over thousands of years.

The positions of continental land masses, ocean currents, and high mountain ranges also affect the pattern of climate zones. Land heats up and cools down faster than water. Many coastal areas have climates affected by wet onshore winds that bring rain, dry offshore winds that create coastal deserts, or reversing winds (monsoons) that cause alternating wet and dry
seasons. Warm ocean currents keep some regions that are far from the equator warm, and cold water upwelling (rising up) from the deep ocean cools some tropical coastlines and islands.

Winds that flow from the oceans onto land generally lose their moisture as they travel inland or uphill. The interiors of large continents like Asia, Australia, and North America are generally dry. When moist air reaches a tall mountain range, it drops rain as it rises and cools. The slopes of mountain ranges exposed to the wind are typically wetter than the sides away from the wind. Arid deserts and semiarid grasslands form in the rainshadows (an area of decreased precipitation on the downwind side of a mountain), behind tall mountains. In the United States, high winds called the jet stream carry moisture-rich air from west to east. It is rainy in the Pacific Northwest, and snowy on the western slopes of the Cascade, Sierra Nevada, and Rocky Mountains. The air is bone dry when it reaches the Mojave, Sonoran, and Chihuahuan deserts of the American Southwest, and northern Mexico.

The Santa Anas are hot, dry air winds that blow from the northeast down the canyons of Southern California. Southern California has a Mediterranean climate. Like the residents of the Mediterranean coast of Europe, southern Californians enjoy extremely pleasant year-round weather. Sunshine is the norm. Balmy onshore winds bring moisture to citrus, olive, avocado, and palm trees that flourish even though it rarely rains. The dry heat and blustery gusts that accompany the Santa Anas are particularly unpleasant for laid-back southern Californians.

Santa Anas usually appear during Fall when a low pressure system of cold air forms over the desert and mountains to the northeast. Because cool air sinks, the air mass moves down toward the coast of Southern California. It warms as it falls, but stays dry. It speeds up as it descends through the canyons and emerges as strong, hot offshore breeze that lasts for a few weeks before abating. Other coastlines experience similar winds, including the Mistral in the Mediterranean and Papagayo in Central America.

The Santa Anas cause a great deal of actual damage. They dry out the coastal vegetation and, once brush fires have ignited, they fan and spread the flames. Because of residential development in the canyons and mountain foothills of southern California, recent Santa Ana fires have threatened many lives and property.

Changes in the factors that determine climate can lead to global climate change over time. Cooling leads to global sea level fall and glacial advance (increased ice formation and spread of ice at the polar ice caps); warming melts the polar ice caps and water rises to cover the edges of the continents. In either case, Earth's climate zones and regional subclimates must adjust to the new temperature and rainfall patterns. Geologic data confirms that Earth has warmed and cooled throughout its history as its position has changed relative to the Sun. Plate tectonic forces (the bumping together and moving apart of large plates of Earth's crust) have rearranged the continents, changing the paths of ocean currents and the pattern of dry land, bodies of water, and ice. The amounts of greenhouse gases and ozone in
the atmosphere have changed naturally and because of human activities. Global warming is the increase in the average temperature of the Earth's surface. Many scientists and environmentalists are concerned that increased carbon dioxide in the atmosphere from the use of fossil fuels like oil and coal could lead to global warming and climate change.

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